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CONTBIBUTIONS TO THE KNOWLEDGE 



the mixture will be able to take up an additional three cubic 

 inches of hydrocarbon vapour, and the illuminating power of 

 the 206 cubic inches will then become 118; thus the hydro- 

 gen produces a gain in illuminating power equal to 9 cubic 

 inches of olefiant gas, or nearly 4*5 per cent, upon the volume 

 of mixed gases. When we consider that coal naptha con- 

 tains hydrocarbons of great volatility, and that these are the 

 surplus remaining after the saturation of the gas from which 

 they have condensed, the importance of this function of the 

 non-illuminating class of combustible gases will be sufficiently 

 evident. I may here remark that incombustible gases could 

 not be employed for this purpose, since their cooling influence 

 upon the flame during the subsequent burning of the gas, 

 would diminish the light to a far greater extent than the 

 hydrocarbon vapour could increase it. 



It is evident that all the three non-illuminating gases form- 

 ing the second class, would perform both the offices I have 

 assigned to them perfectly well, and therefore we have as 

 yet seen no reason for giving our preference in favour of any 

 one of these diluents ; if, however, we study their behaviour 

 during combustion, we shall find that where the gas is to be 

 used for illuminating purposes, hydrogen has qualities which 

 give it a very decided preference over tlie other two. When 

 gas is used for lighting the interior of public buildings and 

 private houses, it is very desirable that it should deteriorate 

 the air as little as possible, or in other words, it should con- 

 sume as small a quantity of oxygen, and generate as little 

 carbonic acid as possible ; and the oppressive heat which is 

 so frequently felt in apartments lighted with gas will also be 

 admitted by all to show the advantage of that gas generating 

 a minimum amount of heat. 



The following is a comparison of the properties of the 

 three non-illuminating gases, in reference to the points just 

 mentioned : — 



1 cubic foot of light carburetted hydrogen, at 60^F. and 



