Whenever decantatiota, or pouring off from the sediment, can be. substituted (as it very frequently may in alcoholic and ethereal di gestions) for filtration, it is to be preferred, as some loss is unavoidably occasioned by every filtration ; whereas by decantation the preci 1, pitate may be dried in the vessel, and if this have been previously counterpoised we can ascertain the weight of adhering matter with great exactness. By this method the precipitate may be as perfectly washed as when filtration is adopted ; the fluid may be poured off very I close to the solid matter, which may again be I diffused through a fiesh pqrtion of washing fluid, again allowed to subside, and the wash ! ling repeated as often as may be necessary. In ' !pouring a fluid from one vessel to another, loss t is avoided by moistening a glass -rod in the liquid, bringing it into cTuitact with the lip of the glass or basin, and pouring the liquid down this rod, which is not removed until the side of the vessel is nearly restored to the vertical position; by observing this precaution we escape the risk of losing any portion from its running down the outside of the vessel. The drops adhering
to the rod are washed into the rest of the solution.
The animal substances that we have to ex amine naturally arrange themselves into fluids and solids, and as this division is convenient in a practical point of view, I shall here adopt it, beginning with those presented to us in a fluid state.
In order to fix some definite limit to our enquiries, those principles only will here be noticed, which', from the frequency of their occurrence or their importance as constituents or products of the living frame,are most likely to be the special objects of attention to the phygician and pathologist. In this number among the fluid products I shall enumerate fibrin, albumen, casein, fatty matters, urea, sugar, the uric, urobenzoic, lactic, and oxalic acids, mentioning a few other compounds, as ptyalin, sulphocyanic acid, &c. &c., vvheu describing the general plan of analysing the secretions in which they Most commonly occur.