BY DR. LAUDER BRUNTON. 567 



is placed in a triangle of platinum wires stretched on a larger one of 

 iron wire (fig. :>42), and heated over a Bunsen's lamp. The cover 

 should be laid on the crucible at first to prevent any loss, and the heat 

 raised very gradually. The cover may be removed during part of the 

 process to allow freer access of air, but towards the end it should again 

 be replaced so that the heat within the crucible may become greater. 

 With the same view, the blowpipe flame; may be substituted tor that 

 of the Bunsen's burner. The crucible is then allowed to cool somewhat 

 on the triangle, but while still warm must be placed over sulphuric 

 acid, and left there till cold. The weight of ash left by a good filter is 

 very inconsiderable ; but it may be ascertained by burning a dozen 

 filters and dividing the weight of the ash by the number. Filters may 

 be almost completely deprived of ash by extracting them with dilute- 

 hydrochloric acid, and washing them with vfciter till the acid reaction 

 completely disappears. 



215. Weighing. The balances most useful in a physiological labora- 

 tory are a fine analytical balance to carry 100 grammes in each pan, and 

 turn easily with half a milligramme or less, and a large balance to carry 

 seventy kilogrammes, and turn with a few grammes. Fine balances 

 are always protected by glass covers, to prevent the access of dust and 

 protect the instrument from draughts of air, etc. Inside this, a vessel 

 containing chloride of calcium is often placed to keep the air dry. The 

 doors of the case should be only opened when the substance or weights 

 are to be adjusted, and should be closed while the beam is o dating. 

 It is convenient to lay the weights cr n. sheet of paper on e floor of 

 the balance, and to mark the weight of e^ . that part 01 the paper 

 where it lies. They must never be touched with the fingers, only with 

 forceps. It is advisable always to place the weights in the same pan 

 (the right) of the balance, and the substance to be weighed in the 

 other. The placing of heavy weights on a fine balance should be 

 avoided, even though they may not exceed the weight which the instru- 

 ment is constructed to carry. Nothing should be placed on the pans or 

 taken from them while the beam is oscillating. It is not necessary to 

 wait each time till the index stops moving in order to see whether there 

 is any difference between the weights in the pans ; for this is ascer- 

 tained much more axactly by observing whether the index oscillates 

 farther on one side of the zero mark th.in on the other, than by noticing 

 its position when at rest. After weighing, add together the weights 

 which are absent from their places on the paper. Note down the weight 

 at once, and check it by adding the weights together as they are lifted 

 from the pan and replaced. No weight should ever be allowed to remain 

 on the balance after weighing. Substances are generally weighed in 

 watch-glasses, small crucibles or small flasks. These may be either 

 weighed separately, and their weight deducted from the total weight, or 

 they may be counterpoised. To save the trouble of weighing them each 

 time, they may be carefully weighed once for all, and their weight noted 

 and marked on them with a diamond, or, if they are of porcelain, in 

 ink. When a crucible with its lid is used, it is usual to put correspond- 

 ing marks on the crucible and its lid, so that the same may be used each 

 time. Counterpoises may be made in various ways. The most con- 

 venient is to choose a piece of brass of about the size of the brass weight 

 which corresponds most closely to the weight of the vessel to be coun- 

 terpoised, and reduce it by careful filing till the weights are exactly 

 equal. If only required for temporary use, a pill-box partly filled with 

 small shot will suffice. 



216. Specific Gravity. The specific gravity of a solid or liquid is 

 its weight compared with that of an equal bulk of distilled water. 

 AY liter and other liquids, however, shrink when cooled, and expand 



