The unit of each denomination being ten times as great as the preceding one. The sub-multiples of the units are characterised by Latin prefixes; thus,

Here the unit of each denomination is ten times less than the one below it.

The following tables give the French Measures of Weight and Capacity, and their English equivalents calculated out to many places of decimals:

For all practical purposes, however, it is sufficient

employed is enclosed in a case fitted with glass doors (Fig. 22). It carries 100 grammes, and indicates a variation of milligramme or grain; this variation is ascertained by the pointer moving decidedly one side or other of the central mark on the register; when in equilibrium it vibrates an equal distance on each side of the central mark. When at rest, or whilst the weights are being adjusted, the instrument is kept at rest by means of a contrivance which steadies the pans; this contrivance is worked by a screw outside the case. The rider (b)—to weigh below or grms. we use the rider; this is formed of bent wire capable of being shifted to any divisions in the beam-the rider being placed on the last, or tenth division, is equal to O grm.; while each division nearer the fulcrum

makes a difference in weight of oor grm. less. The rider is caught up and moved by a brass rod worked outside the case (a).

EXAMPLES OF GRAVIMETRIC OPERATIONS.

Ex. I. Weighing.-Suppose we wish to determine the weight of a platinum basin. We first of all ascertain if the balance is in equilibrium by liberating the pans with the screw and observing if the pointer swings to an equal distance on each side the central mark on the scale; if it does not swing to an equal distance but moves to one side more than the other, the balance must be adjusted. A brass arm about two inches long fixed to a screw above the fulcrum serves for this purpose. A deflection to either side makes that the heaviest.

The pans are steadied and the basin, perfectly cleaned and dried previously in an air-bath, is placed in one pan,

usually the one on the left hand of the operator, and the weights placed in the other; at first we guess at the probable weight, and place say fifty grammes on this pan; this proves too much, for the pointer travels towards the pan supporting the basin. We remove the 50 and try the next highest or 20 grammes; as this is too little we allow it to remain and add the two 10 gramme weights, thus making 40 grammes in all. This being still insufficient we add 5 grammes; still too little; allow it to remain and add 2 grammes ; too little ; add 1 gramme, too much; remove the one gramme weight and try 5 gramme; too much; try 2 gramme, too little; allow 2 gramme to remain and add 1 gramme; too little; add another 'I gramme; still too little. As we had previously tried the 5 gramme weight and found it too heavy, we recommence with '05 gramme; this proving too little is allowed to remain and 02 gramme added; still too little; or gramme, too much; remove and recommence with rider say on 5 ; too much; on 3; this is just sufficient, the pointer swings an equal number of degrees on either side of the scale. The weight of the basin is therefore 47 453 grammes.

Ex. 2. Estimation of the Water, Organic Matter, and Salts. Take a clean platinum capsule, and having weighed it by the above process, place in it a definite quantity of the substance to be determined, say 50 grammes of blood, milk, or finely minced bone or muscle, and evaporate'

I

Evaporation Water-bath.-For ordinary purposes a cheap and efficient water-bath can be made from an old oil can, by fitting it with a cork perforated to admit an ordinary glass funnel. The can is filled with water and the platinum capsule placed in the open part of the funnel, the escape of steam being allowed by a folded piece of paper placed between the edge of the filter and the capsule. The apparatus is supported on an ordinary retort stand and ebullition maintained on an argand gas-lamp placed below. The evaporation requires about two hours. Five or ten minutes must elapse after the water appears dissipated, to allow the residue to become thoroughly dry. The basin is then removed, the bottom wiped quite dry and allowed to cool under a glass shade in which is placed some oil of vitriol in a beaker, in order to prevent absorption of moisture. It is then weighed. The difference is usually very small, and varies from '02 grm. to 'I grm.

over a water-bath till it ceases to lose weight. On weighing when cold the loss will represent the amount of water withdrawn from 50 grammes of the substance; for example, if the united weight of the capsule and 50 grammes of the substance is 110 grammes, before evaporation, and only 70'5 after, the substance will have lost 39'5 grammes of water; and 39'5 × 1000 = 790 grammes

50

the quantity of water in 1000 parts.

The residue left in the platinum capsule, and which by the above weighing was found to weigh 70'5 grammes, is introduced into a muffle furnace or over a Bunsen's lamp, and incinerated till all the organic matter disappears; this residue is again weighed, and its weight represents the amount of inorganic residue in 50 grammes of the substance. If, for example, after incineration the weight of the capsule and residue is 60'4 grammes, then 10'I grammes of organic matter has been burnt off, and 0'4 gramme represents the inorganic residue of 50 grammes of the substance, therefore 0'4 X 1000 = 8 50 grammes the inorganic residue in 1000 parts of the substance.

Ex. 3. Determination of Uric Acid in Urine.-Collect the urine passed in the twenty-four hours and measure. Take 200 c.c. and add 20 c.c. of strong hydrochloric acid. Set aside in a tall urine-glass for twenty-four hours to allow the uric acid crystals to separate. Dry a small filter paper in the air-bath at 120° C. and weigh." Collect the crystals on this filter and wash them well with water slightly acidulated, using a wash-bottle. Dry them with the filter in the air-bath and weigh. For example, the weight of the dry filter is o'27 gramme, with the crystals when dried it weighs 42 gramme, therefore the weight of the crystals in 200 c.c. of urine will be o'15 gramme, and if the quantity of urine passed in twenty-four hours be 950 c.c.: then,