272 On the Analysis of Organic Substances. [Ocr. 
to be dried to a temperature of 212 in a vacuum with sulphuric acid. 
For effecting this more easily, ] had the following apparatus made, 
which answers the purpose very effectually, and at the same time 
will be found simple and convenient.—A (Plate XXXIX. fig. 1,) 
is the flat circular plate of an air-pump, on which is placed C, 
a saucer containing sulphuric acid. B a low receiver communicating 
with the inner vessel G by means of the pipe F. H is a brass cap, 
capable of being made air-tight by means of a screw and leather 
collar, having a square nut L adapted to a key by which it may be 
unscrewed, &c. when necessary. ‘lhe outer vessel K contains water, 
which is kept at the boiling temperature by means of the lamp E, 
which slides upon the tube F, and can thus be raised or depressed 
‘at pleasure. The substances to be dried are put into little glass 
vessels I of the shape of buckets, and are placed in the vessel G, 
and removed from it through the aperture H ‘by means of a hooked 
wire. D is astop-cock, which, when the cap H is removed, may 
be turned, and thus the air prevented from entering the receiver B, 
and the trouble consequently saved of being perpetually obliged to 
exhaust the whole apparatus. 
In using this apparatus it is proper to introduce as little superfluous 
water as possible; or if this cannot be avoided, care must be taken 
not to exhaust all at once, but by slow degrees, otherwise ebullition 
will take place, and the substances be forced out of the glass buckets. 
2. For finding the weight of combination, or of an atom of an 
organic compound, no general rule can be given, as the process 
must vary with the nature of the substance. A careful study of the 
ingenious modes pursued by Berzelius will scarcely fail to suggest 
others. It may sometimes be more conveniently done after an in- 
sight has been obtained of the constitution of the substance under 
examination. But upon the whole, it will perhaps be found one of 
the most difficult steps to effect, and sometimes even impossible. 
3. It is a difficult task, and requires great care and nicety, to 
arrive at an accurate knowledge of the quantities of the elements 
entering into an organic compound. The best mode at present 
known is undoubtedly combustion with oxymuriate of potash in an 
apparatus somewhat similar to Berzelius’s.* I have tried this, and 
found it succeed completely. The only objection to it is its being 
rather too complicated ; and in general, perhaps, it will be found 
better to rest satisfied with the knowledge of the quantity of one 
element, and to make separate experiments for each of those whose 
quantity we may wish to ascertain. In aternary compound, carbon 
and hydrogen are. the elements whose quantities are most easily 
found. Perhaps, however, the real quantity of hydrogen will be 
always somewhat larger than indicated, because the gases extricated 
during combustion must necessarily be in the driest possible state, 
* Lhave tried also the ingenious mode adopted by Mr. Porrett in his analysis of 
prussic acid; namely, of adding multiples of oxygen. This, however, though it 
succeeded in that instance, does not scem capable of universal application, 
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