In some cases, as in drying blood, a tempe rature of 230° may be safely used by employing a boiling solution of Rochelle salt as the exte rior bath ; and in the analysis of the bile a heat of even 260° is recommended by Berzelius. Where sugar or urea is present, even a heat of 200° is injurious, and must therefore be avoided. The operation we are now considering appears one of the simplest that the chemist can have to perform, but I have been induced to dwell the longer upon it as it is one from which, without great care, more mistakes arise than from any other, owing to the pertinacity with, which water adheres to most organic substances. The temperature attainable in an open basin over the water bath is much lower than we should, priori, have been led to imagine. I found, for example, that the temperature of some wheat flour thus drying in an open basin was only 144° F., whilst the water in the bath continued steadily at 196° F. When the basin was covered with a piece of paper, a tempe rature of 161° F. was the highest attained, while a thermometer placed in the water of the bath stood at 194° F. With liquids eva plating it rises somewhat higher ; when plain water was evaporated it stood at 164°, the water in the bath being 208'; and in the case of a viscid fluid like yeast, it varied between 176° and 180°, while the bath raised a therinotneter inserted in it to 210°.
It is therefore desirable to have an appa ratus in which we can ensure any given ternpe raturefrom 212° upwards. For this purpose Liebig has contrived a kind of hot-water oven, consisting of a double box of copper ; in the interval between the outer and inner walls, water saline solutions, or oil, may be poured and heated in the usual way. In one side of this box is a door which may be closed when necessary. The interior chamber and its con tents can thus be maintained with certainty at the same temperature as that of the fluid around them. The best plan of proceeding consists in evaporating liquids to apparent dryness upon the open water bath ; and afterwards subjecting the solid residue, when a tempemture of 212° is not injurious, to complete desiccation in Liebig's oven. So long as the material under examination loses weight, the application of heat must be continued.
Capsules of Wedgewood ware or Berlin porcelain are indispensable and one or 0% o small platinum dishes will b'e found most valu able, especially in the evaporation of albu minous fluids, as the dry residue adheres so strongly to the glaze of earthen vessels that a portion of the basin is invariably removed along with the animal matter, which thus ac quires an undue increase of weight, and the surface of the vessel becomes rough and unfit for use from the difficulty of cleaning it. The Berlin porcelain crucibles are excellent vessels for evaporation, as, being fitted with covers, di dry residue may be preserved from absorbin moisture during the operation of weighing,, b exposure to air. It may be worthy of n tice that adhering organic substances may removed from the surface of vessels in wide they have been kept, by digestion in concen trated nitric or sulphuric acid, or else by stron solution of potash.
Incineration of the dry residue is acco plished by taking a counterpoised porcelain platinum capsule with a determinate quantit, say 10 or 12 grains, of the material to be burne and heatina. it over a circular wicked spirit-lam until the as% completely loses its black colou The capsule should at first be covered to pr vent loss by dispersion on the first applicatio of heat; when visible fumes cease to arise cover may be removed to allow freer acc air; as, however, the temperature rises when the vessel is covered, it will oft found advantageous to leave it loosely c and maintain a steady heat ; sufficient air access to consume the carbonaceous Sometimes the ash may be stirred careful] a platinum wire in order to expose it more to the air. When the ash contains al
phosphates, the last traces of carbon are b off with difficulty, as the phosphates fu. protect the unburned particles from the f action of the air. This inconvenience m overcome by moistening the residue (aft capsule has been allowed to cool) with drops of nitric acid, and again igniting, r ing this manceuvre as often as may be sary. A new difficulty, however, arises chlorides are present, as is ahnost alway case; for at a high temperature these sal decomposed by nitric acid, and the r therefore appears to contain less chlorine is really combined with it.
As the alkaline chlorides are, moreover, not completely fixed at a red heat, some loss is not unfrequently sustained by a long continued ignition. We may dispense with the employ ment of nitric acid and avoid loss from volati lization by shortening the period during which a high temperature is kept up, and simply char ring the mass at a low red heat so as to destroy all organic matter ; the black residuum is then digested in water, by which the alkaline salts dissolve, and after well washing, a little weak nitric acid is employed to remove the earthy coinpounds : charcoal alone remains undis solved. By evaporation of the solutions the amount of saline matter is ascertained.
Filtration may be performed on good white unsized blotting paper cut of a size to drop completely within the funnel; before pouring in the fluid to be cleared, the filter should be moistened, if the solution be aqueous, with water; if alcoholic or ethereal, with a few drops of alcohol or ether, as the case may be. In order to ascertain the quantity of dry residue, the preferable plan is to employ two filters pre viously counterpoised one against the other, and to insert one into the other ; 'the excess of weight the inner filter shews when the filtra tion is complete, after both have been carefully dried on the water-bath, will furnish the quan tity of solid matter. To render the filtration more rapid, the apex of the outer filter may be cut off so as to leave only one thickness of paper at the point. Occasionally we may weigh the filter itself and mark its weight in pencil upon it ; the objection to this plan con sists in the difficulty of obtaining the paper always at the same point of dryness. It should be first dried at 2,12°F., placed in a light covered capsule over sulphuric acid, and allowed to cool, then weighed whilst covered from the air. The same care must be observed on again weighing it with the precipitate. Of course in washing an organic precipitate, the same precautions will be required as vvhen pro ceeding with the analysis of a ,mineral. The stream of washing liquid should be speci ally directed to the edges of the filter, which may be known to be sufficiently washed, when by the evaporation of d. drop of the liquid which passes through on a slip of glass, no perceptible stain is produced.