Cooley's Cyclopaedia of Practical Receipts Volume Ii Part 226

To determine the specific gravity as a solid, we weigh it, first in the air, and then in water. In the latter case it loses, of its weight, a quant.i.ty precisely equal to the weight of its own bulk of water; and hence, by comparing this weight with its total weight, we find its specific gravity. The rule is--Divide the total weight by the loss of weight in water; the quotient is the specific gravity.

The specific gravity of a substance lighter than water may be determined by attaching it to some substance, as a piece of lead, the sp. gr., &c., of which is known. In this way, by deducting the loss in weight of the two substances, when weighed in water, from the loss sustained by the lead alone, when so weighed, we obtain a difference (_a_) which, added to the weight of the substance taken in air (_b_), gives the respective densities. From these the sp. gr. is found by the rule of three:--

(_a_ + _b_) : 1 :: _b_ : _sp. gr._

The specific gravities of substances soluble in water are taken in pure oil of turpentine, rectified spirit, olive oil, or some other liquid, the density of which is exactly known. Sometimes, for rough purposes, the article is covered with a coating of mastic varnish. This last method answers for mercurial pill.

The specific gravity of a substance in fragments, or in powder, may be found by putting a portion (say 100 gr.) into a sp. gr. bottle, filling the latter with distilled water, and then weighing it. The weight of water which it is found to contain, deducted from 1000 (the weight of the bottle when filled with distilled water), gives a difference (_a_) which bears the same relation to the sp. gr. of water (1000) as the weight of the powder (_b_) put into the bottle does to the required sp. gr. Or--

_a_ : 1000 :: _b_ : _sp. gr._

The specific gravity of alloys and mixtures, when no condensation has occurred, is equal to the sum of the weights divided by the sum of the volumes, compared to water reckoned as unity; and is not merely the arithmetical mean between the two numbers denoting the two sp. gr., as is frequently taught. See BEADS (Lovi's), HYDROMETER, MIXTURES (Arithmetic of), &c. For the mode of determining the specific gravity of gases, the reader is referred to the works on chemistry of Miller and Fownes.

The specific gravity of a liquid is found by weighing it in a sp. gr.

bottle, gla.s.s flask, or other vessel of known capacity, and dividing that weight by the weight of the same bulk of water; the quotient is, as before, the specific gravity. A bottle of the capacity of 1000 water-grains (specific gravity bottle) gives the density of a liquid at once, by simply filling it to the given mark, and then accurately weighing it.

We reprint from the 'Journal of the Chemical Society'[179] a new method of determining the specific gravity of liquids, which is said by Dr H.

Sprengel, the chemist who devised it, to be both expeditious and accurate.

Dr Sprengel says:

[Footnote 179: (2) xi, 577.]

[Ill.u.s.tration: FIG. 1.]

"The form of my instrument, as shown in the accompanying fig. 1, is that of an elongated U-tube, the open ends of which terminate in two capillary tubes, which are bent at right angles in opposite directions. The size and weight of this instrument should be adapted to the size and capability of the balance in which it is to be weighed. As our usual balances indicate 1/10 milligram when loaded with 50 grams, the U-tube, when charged with the liquid, should not be heavier than 1000 gr.=(64799 grams).

The instrument which served for my determinations, mentioned below, had a length of 177 cm. (7 inches), and was made of a gla.s.s tube, the outer diameter of which was 11 mm. (7/16 of an inch). It need hardly be mentioned that the U-shape is adopted for the sake of presenting a large surface, and so rendering the instrument sensitive to changes of temperature. The point, however, I wish to notice more particularly (for reasons explained below) is the different calibre of the two capillary tubes. The shorter one is a good deal narrower (at least towards the end) than the longer one, the inner diameter of which is about 1/2 mm. The horizontal part of this wider tube is marked near the bend with a delicate line (_b_). This line, and the extremity of the opposite capillary tube (_a_), are the marks which limit the volume of the liquid to be laid.

The filling of the instrument is easily effected by suction, provided that the little bulb apparatus (as represented in fig. 2) has previously been attached to the _narrow_ capillary tube by means of a perforated stopper, _i. e._ a bit of an india-rubber tube tightly fitting the conical tubules of the bulb. On dipping the wider and longer capillary tube into a liquid, suction applied to the open end of the india-rubber tube will produce a partial vacuum in the apparatus, causing the liquid to enter the U-tube.

As the partial vacuum maintains itself for some time (on account of the bulb, which acts as an air-chamber), it is not necessary to continue the suction if the end of the india-rubber tube be timely closed by compression between the fingers. When bulb and U-tube have about equal capacity, it is hardly necessary during the filling to repeat the exhaustion more than once.

Without such a bulb the filling of the U-tube through these fine capillary tubes is found somewhat tiresome; the emptying the U-tube is effected by reversing the action, and so compressing the air. After the U-tube has been filled, it is detached from the bulb, placed in water of the standard temperature almost up to the bends of the capillary tubes, left there until it has a.s.sumed this temperature, and after a careful adjustment of the volume, is taken out, dried, and weighed.

Particular care must be taken to ensure the correctness of the standard temperature, for a mistake of 01 causes the weight of 10 c.c. of water to be estimated either too high or too low by 014 milligram, giving rise to an error in the fifth decimal, or making 100,000 parts 1000014 parts.

These determinations have been made in Dupre's apparatus, which, when furnished with a sensitive thermometer, allows the fluctuations of temperature to be fixed within the limits of 001. If many determinations had to be made, I should avail myself of Scheibler's ('Zeitschrift fur a.n.a.lytische Chemie,' vol. vii, p. 88, 1868) electro-magnetic regulator for maintaining a constant temperature.

A peculiar feature of my instrument is the ease and precision with which the measurement of the liquid can be adjusted at the moment it has taken the standard temperature; for it will be found that the liquid expands and contracts only in the wider capillary tube, viz. in the direction of the least resistance. The narrow capillary tube remains always completely filled. Supposing the liquid reaches beyond the mark _b_, it may be reduced through capillary force by touching the point _a_ with a little roll of filtering paper. Supposing, however, that in so doing too much liquid is abstracted, capillary force will redress the fault if point _a_ be touched with a drop of the liquid under examination; for this gentle force acts instantly through the whole ma.s.s of the liquid, causing it to move forward again to or beyond the mark.

As the instrument itself possesses the properties of a delicate thermometer, the time when it has reached the standard temperature of the bath may be learned from the stability of the thread of liquid inside the wider tube. The length of this thread remains constant after the lapse of about _five minutes_.

In wiping the instrument (after its removal from the bath) care should be taken not to touch point _a_, as capillarity might extract some of the liquid; otherwise the handling of the liquid requires no especial precaution.

The capillary tubes need not be closed for the purpose of arresting evaporation, at least that of water. I have learned from the mean of several determinations that the error arising from this source amounts in one hour to 1/20 of a milligram.

In cases where the temperature of the balance-room is high, and the expansion coefficient of the liquid to be examined is considerable, it may be found necessary to put a small cap (bead-shaped and open at both ends) over the extremity of the _wider_ capillary tube, for the purpose of retaining the liquid, which during the time of weighing might otherwise be lost, owing to its expansion. When a cap is used the _wider_ capillary tube need not be longer than the narrow one.[180]

[Footnote 180: This instrument is manufactured by E. Cetti & Co., 11, Brooke Street, Holborn, London.]

The 'Compte Rendus'[181] describes a new specific gravity apparatus, the invention of M. Pisani. The apparatus in question consists of a gla.s.s vessel about 5 c.c. capacity, closed with a perforated stopper like an ordinary specific gravity bottle. To the side of the vessel is joined a tube, coming off at an angle of about 45, about 25 cm. long, and 4 mm.

internal diameter, and graduated at 50ths of a c.c. The vessel is filled with water, the level of which is read off in the tube held vertically, the finger being held over the hole in the stopper; 2 or 3 grams of a mineral are then placed in the flask, the stopper is replaced, care being taken to lose no water, and the level is again read off in the graduated tube, held vertically as before. The difference in the two readings gives the volume of the mineral taken.

[Footnote 181: lx.x.xvi, 350-352 ('Journ. Chem. Soc.')]

[Ill.u.s.tration]

=SPEC'TACLES.= See EYE, VISION, &c.

=SPEC'TROSCOPE.= An instrument devised for examining the spectra of flames. (See _below_.)

=SPEC'TRUM a.n.a.l'YSIS.= See a.n.a.lYSIS, SPECTRUM.

=SPEC'ULUM MET'AL.= _Prep._ 1. Take of copper, 64 parts; pure tin, 69 parts; melt them separately under a little black flux; next incorporate them thoroughly by stirring with a wooden spatula, and run the metal into the moulds, so that the face of the intended mirror may be downwards; lastly, allow the whole to cool very slowly.

2. Pure copper, 2 parts; pure tin, 1 part. Used to make the mirrors of reflecting telescopes. The addition of a little metallic a.r.s.enic renders it whiter.

=SPEL'TER.= See ZINC.

=SPERMACE'TI.= _Syn._ CETACEUM (B. P., Ph. L., E., & D.), L. A concretion prepared from the oily matter of the head of the _Physeter macrocephalus_, or spermaceti whale. It is demulcent and emollient; chiefly used in ointments and cerates.

=SPHEROID'AL STATE.= It is found that water, or any other volatile liquid, thrown on a metallic plate heated to dull redness, is not resolved into vapour, but, a.s.suming a somewhat globular form, remains intact, until the temperature becomes sufficiently lowered to allow of contact between the liquid and the heated surface. It is then immediately volatilised. M.

Boutigny, who fully investigated this subject, has also shown that the same thing happens when a solid body containing water is subst.i.tuted for the liquid in the above and similar experiments. Thus, the finger or hand, under certain restrictions, may be thrust, with perfect impunity, into a stream of molten metal, and ice may be produced by throwing water into a red-hot crucible. This last experiment, as performed by MM. Boutigny and Prevostaye, is essentially as follows:--A thick platinum crucible, of the capacity of 1 fl. oz., is heated to redness over a powerful spirit lamp, and some liquid anhydrous sulphurous acid (a very volatile substance) poured into it by means of a pipette; the acid a.s.sumes a spheroidal form, and does not evaporate; a few drops of water are now introduced into the sulphurous acid in the same way; the diluted and slightly cooled acid instantly flashes off in vapour, and, robbing the water of its caloric, leaves the latter in a frozen state; and, if the operator seizes the right moment, a solid lump of ice may be thrown out of the red-hot crucible.

By subst.i.tuting for anhydrous sulphurous acid a mixture of solid carbonic anhydride and ether, and for water a few grains of quicksilver, this latter may be reduced to the solid condition, and may be turned out of the red-hot crucible in the form of a small frozen ma.s.s.

The spheroidal condition of "liquids is a complicated result of at least four distinct causes. Of these the most influential is the repulsive force which heat exerts between objects which are closely approximated towards each other. When the temperature reaches a certain point actual repulsion between the particles ensues.

"Besides this repulsive action occasioned by heat, the other causes which may be mentioned as tending to produce the a.s.sumption of the spheroidal condition by the liquid are these:--

"1. The temperature of the plate is so high that it immediately converts any liquid that touches it into vapour, upon which the spheroid rests as on a cus.h.i.+on.

"2. This vapour is a bad conductor of heat, and prevents the rapid conduction of heat from the metal to the globule.

"3. The evaporation from the entire surface of the liquid carries off the heat as it arrives, and a.s.sists in keeping the temperature below the points of ebullition. The drop a.s.sumes the spheroidal form as a necessary consequence of the action of cohesion among the particles of the liquid, and the simultaneous action of gravity on the ma.s.s."[182]

Boutigny found that, when a liquid in a state of ebullition was brought into contact with a surface heated to such a degree as to cause the liquid to a.s.sume the spheroidal state, its temperature immediately fell 3 or 4 C. below the boiling point.

All liquids are capable of a.s.suming the spheroidal condition; but, as the temperature necessary for this purpose, varies with the boiling point of each liquid (the lower the boiling point the lower the temperature necessary, and _vice versa_), it follows that the conducting surface requires to be differently heated for each liquid. The exact temperature to which the plate should be heated to produce the spheroidal condition in any liquid, depends partly upon the conducting power of the plate, and partly upon the latent heat of the vapour; the less this is, the more nearly the temperature of the plate approximates to the boiling point of the liquid.

Boutigny believed that the temperature of each liquid, when in the spheroid condition, was as invariable as that of its boiling point; but Boutan has demonstrated that this is a not quite accurate statement, since the temperature of the same liquids, when a.s.suming the spheroidal form, is liable to slight divergence.

The following table, showing the lowest temperature of the plate and the temperature of the spheroid for certain liquids, is given by Boutigny:--

+-----------------------+-----------------------+--------------------------+ Temperature of Plate. Temperature of Spheroid. +-----------+-----------+-------------+------------+ Liquid employed. F. C. F. C. +-----------------------+-----------+-----------+-------------+------------+ Water 340 171 2057 964 Alcohol 273 134 1679 755 Ether 142 61 936 342 Sulphurous anhydride ... ... 131 105 +-----------------------+-----------+-----------+-------------+------------+

Solids may also be made to a.s.sume the spheroidal condition, as when, for instance, some crystals of iodine are thrown upon a red-hot platinum disc, or into a platinum crucible similarly heated.

[Footnote 182: Miller.]