ATTRACTION 179 



hydrogen. By a similar process, the specific gravity of any other 

 gas may be discovered." 



" The apparatus for ascertaining specific gravities, above represent- 

 ed, is that which is recommended by Henry. The gas may be more 

 accurately measured, by using one of the volumeters."* 



" The weight of any given number of cubic inches of air or gas, 

 as one hundred, for instance, may be known by introducing a certain 

 quantity into the globe, as above described, and noticing the acces- 

 sion of weight : then, as the number of cubic inches introduced, is to 

 the weight gained by its introduction, so is one hundred to the weight 

 of one hundred cubic inches of the fluid." 



" The number of cubic inches introduced, may be known by means 

 of the graduation on the receiver, R." 



If there be a column of water or mercury standing in the jar, the 

 gas will be less compressed than if there were no such column. 

 Therefore, the density will be inversely the volume directly as 

 the height of this column. Hence, to ascertain the volume, say 

 H : H h'_ \v : x. Here, H is the height of the barometer, h the 

 height of the column,f v the observed volume, and x the volume re- 

 quired. 



In weighing the gases in order that the result may be correct, the 

 gas should be pure ; it should be dry, or due allowance should be 

 made for watery vapor, and if the experiment is not made when the 

 barometer is at 30 inches, and the thermometer at 60, the observ- 

 ed volume should be reduced by calculation, to what it would be, at 

 the medium temperature J and pressure. 



The purity must be secured and ascertained by the modes appro- 

 priate to each particular gas. 



Moisture must be removed, as far as possible, by exposure to dry 

 muriate of lime, quick lime recently ignited, or fused potash ; or other 

 substances that powerfully attract water. 



For temperature ; the volumft of a gas is as the temperature direct- 

 ly, and as operations on gases are almost always carried on above 32, 

 we first ascertain the volume that the gas would occupy at that tem- 

 perature, which is done by multiplying the total volume by|| 480, 

 and dividing the product by|| 480, -f- the number of degrees that the 



* See Dr. Hare's Compendium. 



t The column being of mercury, or due allowance made if it is water; a foot of 

 water representing nearly an inch of mercury. 



| Gloves should be worn while handling the vessels, or they should be lifted by 

 the keys of the stop cocks, that the warmth of the hands may not cause expansion in 

 the gas. 



For a general formula, see Henry, 6th Ed. Vol. I, p. 25, and Turner, 2d Ed. Vol. 

 I, p. 71. 



[| Because a gas expands ^ j^ part of its volume by every degree of heat. 



