104 MAGIC LANTERN WITH THE LIME LIGHT [Cn. IV 



For example, suppose the pressure gauge indicates 1800 Ibs. per 

 square inch, then it would be under a pressure of 1800 -r- 15 = 120 

 atmospheres. 



If the pressure gauge should read 100 kilograms per square 

 centimeter, then it would be under approximately 100 atmospheres 

 pressure. 



From Boyle's law of the relation between the volume of a gas 

 and the pressure to which it is subjected it is known that if one 

 starts with a cylinder holding five liters of oxygen or hydrogen, or 

 indeed of any other gas, under one atmosphere pressure, it will hold 

 twice as much under two atmospheres, etc., so that a cylinder of 

 five liters capacity at one atmosphere, will hold 500 liters at 100 

 atmospheres pressure. Now to determine the amount of gas 

 present in a given cylinder with the high pressure gauge one must 

 know the capacity of the cylinder under the ordinary atmospheric 

 pressure, and multiply this amount by the number of atmospheres 

 of pressure indicated on the gauge. For example, if the capacity of 

 the cylinder is i o liters at one atmosphere (often called no pressure) 

 and the high pressure gauge indicates that the gas in the cylinder 

 is under a pressure of 25 atmospheres, then the amount of gas is 

 10 X 25 = 250 liters of gas; and so in like manner with any other 

 pressure. For example, in England, the cylinders are filled under 

 120 atmospheres pressure; this would give in the above case 

 10 X 120 = 1200 liters to the full cylinder. 



On the Continent, the filling pressure is often 100 atmospheres 

 and the cylinder of the same capacity would then contain 10 X 100 

 = 1000 liters of the gas. 



The practical application of this knowledge is to determine in a 

 given case whether there is sufficient of the gases present for the 

 exhibition. Authors differ somewhat in estimating the amount 

 of gas used per hour with the lime light lantern. A conservative 

 estimate would be, for oxygen, 85 liters (about three cubic feet) 

 and, for hydrogen, something over twice that volume, as, in prac- 

 tice, there is an excess of hydrogen ( 161). 



157. Limes. The masses of unslaked lime (calcium oxid) 

 used for the lime light are usuallv cylindrical in form. For some 



