CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 103 



In Great Britain and on the Continent oxygen cylinders are, by 

 common usage, painted black, and the screw threads are right- 

 handed. 



Hydrogen cylinders are painted red, and their screw threads 

 are left-handed. 



In the United States of America this uniformity of color and dis- 

 tinction of screw threads is not always found. 



155. Hydrogen in steel cylinders. Hydrogen gas is also 

 compressed in steel cylinders, and forms an article of commerce. 

 It must also be drawn off through a pressure reducing valve. 



Every precaution should be taken to avoid mixing the two gases 

 in large quantities. Safety lies in mixing the gases only at the 

 moment of exit from the two tubes of the blow-through jet or in 

 the small mixing chamber of the mixed jet. 



156. Pressure gauges for gas cylinders. While a pressure 

 reducing valve is a practical necessity, the pressure gauges are 

 highly desirable. 



The one beyond the pressure reducing valve is a low pressure 

 gauge and may indicate the pressure in millimeters of mercury or 

 in centimeters of water (or, of course, in inches of water or mercury). 

 This shows the pressure under which the gas is actually being used. 



The gauge next the cylinder registers the full pressure within. 

 The figures on the dial usually represent atmospheres of pressure, 

 one atmosphere being 760 mm. of mercury. The special purpose 

 of this gauge is to enable one to determine the amount of gas in the 

 cylinder at any given time, hence it is sometimes called a "capacity 

 meter" or a "finimeter." 



If the pressure gauge does not indicate directly the atmospheric 

 pressure, it may give the number of pounds per square inch or the 

 number of kilograms per square centimeter. To change these to 

 atmospheres one can use the approximate values: 15 Ibs. per 

 square inch = i atmosphere; or i kilo per square centimeter = i 

 atmosphere ( is6a). 



156a. The exact values are: 



One atmosphere equals 14.73 pounds per square inch. 



One atmosphere equals 1.033 kilograms per square centimeter. 



