1820.] Scientific Intelligence. 233 



stop-cocks connected with the apparatus before the gas is let ia 

 All the precaution necessary is to take care that no particles 

 of water or mercury (supposing the gas to be standing over 

 mercury) insinuate themselves into the flask. It is obvious that 

 the volume of gas which will enter the flask will be precisely 

 equal to the volume of common air that has been previously 

 drawn out of it by the air-pump. I now weigh the flask thus 

 filled with the gas whose specific gravity I wish to know. The 

 increase of weight of the flask above its weight when exhausted 

 gives exactly the weight of the gas introduced into the flask. 

 Let this weight be = b. 



We have now obtained the weight i of a certain unknown 

 volume of gas^ and the weight a of exactly the same volume of 

 common air. Now I say that the specific gravity of the gas is 



= -. Or we have only to divide the weight of the gas h by the 



weight of the air a. This quotient is the true specific gravity of 

 the gas without any correction whatever being requisite either 

 for temperature, or for the height of the barometer ; because all 

 gaseous bodies imdergo the same change of volume by the same 

 application of heat or pressure. Hence the specific gravity of 

 air bears the same ratio to that of the gases at every temperature, 

 and under every pressure. 



Had Berzelius employed this method, he would not have 

 required three days to obtain a very imperfect approximation to 

 the specific gravity of sulphurous acid. Whoever will take the 

 specific gravity of this gas in the way that I have directed above 

 will find it just double the specific gravity of oxygen gas ; or 

 2-2222. 



It very seldom happens that the gas whose specific gravity is 

 taken is perfectly free from some admixture ofconmion air. In 

 such cases it is always necessary to determine the volume of air 

 in the gas, and when this is known, the specific gravity of the 

 pure gas may be deduced from that of the mixture by a very 

 •simple calculation. Let x = specific gravity of the pure gas, 

 A = the volume of air in the mixture, a = specific gravity of 

 air; B = volume of pure gas present, c = specific gravity of the 

 mixed gas, then 



(A + B)c- A a 



X — 



li 



VI. Combinations of Prussic Acid. 



My facetious correspondent, who subscribes himself " Jack 

 Addle," would have spared his animadversions on Count Le 

 Maistre's paper, if he had recollected that mistaken inferences 

 in science, when the mistake is obvious, and has been pointed 

 out even before it has been committed, can never be in the least 

 injurious to the progress of science ; while a haughty rejection 

 of the first attempts at experiment, even though these attempts 



