198 LECTURES TO SCIENCE TEACHERS. 



A few words may be useful to you as to the size and the 

 nature of the jets to be employed ; and then I shall show, 

 in conclusion, two or three applications of these sensitive 

 flames. The first form of jet that I employed was a simple 

 piece of glass tubing drawn out to a point, and then filed 

 into a Y-shape. Buch a jet yields a flame which is 

 capable of being affected by a whistle, and has the advan- 

 tage of being sensitive under the ordinary pressure of gas. 

 Here is a similar jet made out of a piece of brass tubing. 

 You see that if I whistle to this name it spreads out 

 sideways. This is at the ordinary pressure. When the 

 pressure of gas increases, as it does towards the evening, 

 the effect is still better. A brass circular orifice also 

 yields a very good sensitive flame under considerable pres- 

 sure. But the most sensitive kind of flame is obtained by 

 allowing the gas to stream through a circular orifice made 

 of steatite such as is used in the jet photometers. It was, 

 in fact, when noticing the influence of sound upon one of 

 these jet photometers, that I was led to use the sub- 

 stance. The jet photometer burner will yield a very tall 

 tapering flame of gas some two feet high. If the gas 

 be very rich in quality the flame will be much higher than 

 if the gas be poor, and the higher you make your flame the 

 more sensitive it is. Hence a sensitive flame may be 

 employed as an extremely delicate test of quality of 

 gas. You will find that if a burner admits No. 19 

 wire (Birmingham, wire gauge), it requires a pressure of 

 three and-a-half inches of water to bring it to its most 

 sensitive condition. If it admits No. 21, which is 

 a smaller size, it requires six or seven inches of water to 

 bring it to its most sensitive condition. This pressure is 

 best obtained by using the gas from a gasholder and not 

 from a gas-bag. The gas-bag has the disadvantage of 

 continuously varying in its pressure, and, moreover, there 

 are vibrations in the gas-bag itself which are extremely 

 detrimental to the effect sought. The gasholder ought 

 to be one yielding a very steady flow of gas. I have 

 fitted up in my laboratory a gasholder for this purpose. 



There is also another point of very considerable import- 

 ance in connection with the subject, namely, having the gas 

 way perfectly free. The gas-cock should not be partly 

 turned. If it is so the gas in passing through it will be subject 



