224 Mr. Porrett’s experiments relative to the 
3d. That of determining with great accuracy, the quantitv 
of oxygen which combines with the elements of the prussic 
acid during its combustion, so as after allowing for what has 
been expended in the formation of carbonic acid, to be able to 
infer with confidence, from the disappearance of the rest, the 
quantity of hydrogen which was contained in the acid. 
The property which the prussic acid possesses of assuming 
the liquid form at a low temperature, and that of a gas or 
vapour at common temperatures, the volume of which is 
materially influenced by mixture with other gases, and by 
slight alterations of temperature and pressure ; did not appear 
to me to be favourable to the employment of it in an uncom- 
bined form for the purpose of its analysis. 
I therefore determined upon employing it in the state of 
condensation in which it exists in prussiate of mercury, and 
this determination made me undertake the analysis I have 
just described of that salt; of the correctness of which, hav- 
ing satisfied myself, I conceived that I had overcome the 
first difficulty. 
The second and third difficulties I thought would be best 
surmounted by employing, for the combustion of the prussic 
acid, the same oxide with which it is united in the prussiate 
of mercury, namely, the red oxide of that metal ; increasing 
the quantity of it by multiples of that which the salt contains, 
until I found that the whole of the prussic acid was decom- 
posed. 
I made a number of experiments upon this plan, the results 
of which proved to me that the quantities of carbonic acid 
and of azote gases produced, did not arrive at the maximum 
until five times the quantity of red oxide of mercury contained 
