I.-PROX I MATE ANALYSIS.
As the limits of this article preclude the pos i;bility of my entering into detail upon the !rdinary operations of analysis, a task happily .nidered unnecessary by the excellent manuals 'e possess on the subject, I shall limit myself .) a few remarks on processes connected more nmediately with organic analysis.
, It is needless here to insist upon, the impor nce of scrupulous attention to the purity of 1:e re-agents employed, as it is a precaution .fficiently obvious. These re-agents are few number : sulphuric, nitric, hydrochloric (or muriatic), and acetic acids, solutions of potash, ammonia, and carbonate of ammonia, alcohol, and ether, constitute the most important; if to these we add solutions of Chloride barium, Acetate lead, N itrate silver, Subacetate lead, Oxalate ammonia, Sulphate copper, Phosphate soda, Sesquichloride iron, Ferrocyanide potas- Bichloride platinum, siuni, Tincture galls, Alum, Hydrosulphuret am Lime water, monia, with a blowpipe, platinum foil, spirit-lamp, forceps, test-tubes and a stand for them, a.few watch-glasses, evaporating dishes and Florence flasks, a retort stand, funnels of different sizes, filtering paper and some lipped glasses, with pieces of glass rod and strips of window glass, we shall be tolerably well prepared for the operations of proximate analysis. Of course distilled water must always be employed in analytical enquiries.
For proximate analysis scales weighing 2000 grains ancl turning with 40th of a grain, when fully loaded, will be sufficient ; but for ulti mate analysis they should be sensible to ihth of a grain when each pan carries 1000 grains.
When the weight of a 'dry residue is to be ascertained; the object is attained with most accuracy by first counterpoising the vessel when empty, and then determining the increase of weight after the desiccation is completed.
The desiccation of all organic substances is best performed, where practicable, in the ex hausted receiver of an air-pump, over sulphuric acid, by Leslie's process : a flat dish of oil of vitriol is placed on the plate of the pump, and the substance to be dried supported above it in a basin by a triangular framework of vvire; the air is exhausted, and care taken to maintain a good vacuum ; the residue thus procured is always much purer irnd whiter than that fur nished by any other' means, but it is a tedious and circuitops process, and requires ten days or a fortnight for its completion. Upon this ac count, and for other reasons, this method can not generally be adopted. The plan which, next to it, presents the fewest objections, con sists in evaporating by a steam or water heat, so that the temperature can never exceed 212° Fahr. Various methods may be resorted to
for effecting this object ; by placing one basin within another containing water, an extempo raneous bath is procured ; but the end is more conveniently attained by the employinent of a shallow box of copper, zinc, or tin plate, in the top of which are half-a-dozen circular apertures of different sizes with projecting vertical rims, upon which lids may be fitted when not in use; any vessel to be heated is placed over one of these apertures, and the temperature maintained by oil, gas, or sand heat.
Perfect desiccation is essential to accuracy, and from the destructible nature of some or ganic compounds, especially under the com bined influences of atmospheric oxygen and an elevated tempemture, it is dangerous to effect it by heat and difficult by any other means. In some delicate experiments the following plan, which combines the application of moderate heat with Leslie's method, already described, tnay be advantageously employed, though the manipulation is rather complicated and trouble some. In a counterpoised retort which will sustain exhaustion, a given weight of the sub stance to be dried is placed and connected with a tubulated receiver containing oil of vitriol, by a sound cork secured on the exterior with several folds of bladder, well soaked before applying it. Through the tubulure of the receiver passes a small glass tube. This junction likewise is ren dered air-tight, with a cork and bladder. The tube, about an inch from the tubulu re,is,previous to its insertion, drawn out and narrowed to a. ayillary bore, so that at pleasure it may be easily drawn off and sealed by a jet of flame from the blowpipe. Matters being thus arranged, the tube from the receiver is united by a connecter of caoutchouc with another tube, and this again %vith the air-pump, and exhaustion is performed. When a sufficient vacuum has been produced, the whole is allowed to stand for an hour. If, at the end of that time, the mercury in the gauge retain its level, the apparatus is air-tight and may be detached from the pump by seal ing the tube proceeding front the receiver in its capillary portion. We may now apply a gentle heat to the bulb of the retort by means of a water bath, or otherwise, and can cool the receiver. Great caution is of course requisite in handling the exhausted vessels, :ts the slightest abrasion of the surface might cause fracture.