COMPRESSED GASES 



this to be the case ; but good cylinders will retain their full 

 charge for a year or more. 



45. Testing Contents of Cylinders. It is exasperating 

 to find no supply of gas at the critical moment, and this 

 makes it important to know how to ascertain the amount 

 really on hand. The opticians use a Bourdon pressure-gauge, 

 which is screwed on by a union -joint, and gives the pressure 

 instantly, then by calculation we get the quantity. Usually 

 the cylinder is ' full ' to its capacity at 1800 Ibs. : if so, the 

 following will be the content, at other pressures, of the sizes 

 commonly in use. 



Constant exhibitors will find a gauge, costing about 21. 10s., 

 well worth the expense, and it is the only method of knowing 

 what quantity of hydrogen or house-gas is available; but 

 where oxygen alone is used, it is sufficient, as was pointed out? 

 in a letter by Mr. C. H. Cathcart, simply koiveigh the cylinder 

 A cubic foot of oxygen weighs as much as l'43oz., or loz=0'7 

 cubic foot nearly. If therefore the cylinder be private pro- 

 perty, let it be weighed when empty, and the weight marked, 

 or a note kept. Deduct this net weight from the weight at 

 any given time, and, 



Difference in oz. x 0'7 = cubic feet of gas. 



Thus, suppose a 12-feet cylinder weighs empty 13lbs. 14^oz., 

 and after an hour's use on one evening, we want to see if 

 there is enough to last again. We weigh it, 141bs. lO^oz. 

 difference 12oz. Then 12oz. xO'7=8'4, or nearly 8^ feet 

 of gas. When a cylinder is not known, we may assume it is 



