2OO HOW TO WORK 



reddened by the addition of a trace of acetic acid, according as the 

 reaction is supposed to be acid or alkaline. 



283. On Filtering. The process of filtration is one which the 

 microscopist as well as the chemist frequently has to perform. To 

 filter a deposit from a solution, in quantity, is easily effected by the 

 use of ordinary filtering paper, folded, pi. XXII, fig. 147, and placed 

 in a small glass funnel, fig. 139, pi. XX H . But sometimes we find 

 it necessary in microscopical analysis, to filter the deposit from a 

 single drop of fluid. This may be effected by cutting a very narrow 

 strip of filtering paper, and bending it into a V-form, upon one of 

 the glass slides. The drop is made to pass between the limbs of the 

 V, and upon inclining the slide, clear fluid will gradually pass through 

 the apex of the V, and can be conducted away to another part of 

 the slide, by a very fine glass rod, where other tests may be 

 applied. 



284. Evaporation and Drying. The evaporation of fluids, and 

 the desiccation of organic solids, must always be conducted over a 

 water-bath, otherwise there is great danger of decomposition occur- 

 ring. For operations upon small quantities, the water-bath repre- 

 sented in pi. XIV, fig. 73, will suffice, or the cans of the injecting 

 apparatus, pi. XXIII, fig. 152, may be removed, and basins placed 

 over the holes. 



In endeavouring to obtain crystals of organic substances, it is 

 always advantageous to evaporate the solution over the surface of 

 sulphuric acid under a bell-jar, pi. XX, fig. 131, or, what is better 

 still, in vacuo, pi. XX, fig. 129. In some instances, the evapora- 

 tion may be conducted by simply exposing the liquid placed in a basin 

 or watch-glass, and covered lightly with paper, to the air ; or, where 

 very slow evaporation is necessary, the watch-glass may be covered 

 over with a bell-glass. 



285. incineration. By incinerating a small portion of any 

 organic substance, upon a piece of platinum foil, or in a platinum or 

 porcelain crucible, we are enabled to ascertain whether it contains 

 inorganic salts, or consists entirely of organic matter, in which case 

 the substance leaves only a black residue, which burns off entirely 

 after a short time. In order to obtain the inorganic constituents per- 

 fectly free from carbon, it is sometimes necessary to keep the mass 

 at a dull red heat for a considerable time. The addition of a drop 

 of nitric acid, causes the rapid oxidation of the carbon. If, 

 however, the temperature be too high, the process may be much 

 retarded, in consequence of the fusion of some of the salts, as the 

 phosphates and chlorides, and the inclusion of small masses of carbon, 

 which are thus protected from the action of the atmosphere. The 



