Miscellanies. 205 



Dr. Mitchell's apparatus, the capacity of the receiver is about one fourth 

 of that of the generator, which is probably so large as to occasion con- 

 siderable loss in this way. 



But it is important also that there should be no unnecessary space in 

 the generator, or in other words, that the materials used to charge it, 

 should very nearly fill it. After introducing the vessel of sulphuric 

 acid and inserting the plug, there must of course remain a little space 

 filled only with air ; and this probably is necessary, for after chemical 

 action takes place, the several substances formed appear to occupy a 

 little more space than before. But if possible, the quantity of soda, 

 &c. used, should be such that the sulphate of soda which is formed, and 

 carbonic acid in the liquid state, should entirely fill the generator. If 

 then the receiver is of the proper capacity, after the liquid acid is dis- 

 tilled over into it, there will remain only the space it previously occu- 

 pied in the generator to contain the gaseous acid, which of course must 

 be lost ; but this is the least loss which the nature of the case admits of. 



When our apparatus was first constructed, (the generator of which 

 holds five pints,) we were accustomed to use at a charge two pounds 

 of the bicarbonate, and sulphuric acid and water in proportion, from 

 which we obtained but a very little of the liquid in the bottom of the 

 receiver ; but upon increasing the quantity to two and a. half pounds of 

 soda, with sulphuric acid, &c. in proportion, we were at first a little sur- 

 prised to see the liquid come over until the receiver Wcis entirely full, 

 and with such rapidity as to leave the impression upon the mind, that 

 more might have been obtained if the receiver had been a little larger. 

 Once or twice only we have made use of two charges of two pounds 

 of soda each, condensing the whole of the liquid acid obtained into the 

 receiver, which, however, was then scarcely filled. If therefore we 

 may put confidence in these results — and we believe they may be relied 

 on — we arrive at this conclusion, that two and a half pounds of the 

 bicarbonate of soda, with sulphuric acid, 6z;c. in proportion, used at a 

 single charge in an apparatus of the capacity of ours, will afford quite 

 as much or more liquid carbonic acid, than five pounds of soda, &c. 

 used at two separate charges. 



The third point mentioned above as requiring attention, is the differ- 

 ence of temperature between the receiver and generator during the 

 distillation of the liquid acid into the receiver. This may be accom- 

 plished either by heating the generator or cooling the receiver, but the 

 last is much the best method. Dr. Torrey informs me that he has 

 sometimes surrounded the receiver with a powerful freezing mixture, 

 with excellent effect. We might even unite the two methods, using 

 proper precautions, but the enormous pressure of the gas increases so 

 rapidly with the increase of temperature, that it can scarcely be con- 

 sidered safe to apply heat to any part of the apparatus. 



