44 SECTION PHYSICS. 



impinging of the sonorous waves on the flame higher up. The action 

 occurs in the orifice. Converging the sonorous waves of a small 

 vibrating reed upon the nipple, a little below the orifice, they produce 

 no effect. Causing the waves to converge higher up, there is no 

 action upon the flame. Converging the waves on the orifice itself, 

 violent action occurs on the flame. We are to make this flame the 

 test of the reflection of sound. When my excellent friend, your 

 Chairman, mentioned the reflection of sound, he meant it to be from 

 the limiting surfaces of gaseous layers, not the ordinary reflection 

 of sound from the surfaces of solids or liquids. This is the pecu- 

 liar feature of the experiments that these echoes will take place at 

 the limiting surfaces of layers of gas. And here, I think, we may 

 confine ourselves, inasmuch as I know there are many gentlemen 

 coming after me, to one illustrative experiment. For the purpose of 

 making that experiment we have this apparatus devised, I may say, 

 by Mr. Cottcrell, and executed by Messrs. Tisley and Spiller. Through 

 a series of apertures, we might throw into this apparatus air saturated 

 with various vapours ; such, for example, as the vapour of ether. We 

 should, in this way, destroy the homogeneous character of the air in 

 this horizontal tube. But instead of making an experiment upon 

 vapours, I pass on to one .representative experiment which will enable 

 you to infer the character of all other experiments made with this 

 apparatus. Underneath this tube are apertures through which, when 

 this series of gas flames are turned on, the columns of heated air 

 from the gas flames will enter the tube. In that way, instead of 

 having a homogeneous horizontal column of air, we obtain a column 

 of air wherein heated layers alternate with cooler layers. The action 

 of this upon the sound will be made manifest to you. At every 

 passage of the sound wave, from the heated layer to the non-heated 

 layer, there is a slight echo, and those echoes occur even in that 

 short distance so frequently as entirely to waste the direct sound in 

 echoes. It is precisely analagous to the action of a cloud or of foam 

 upon light. You have there the mixture of two transparent sub- 

 stances air and water and in virtue of those repeated reflections 

 at the limiting surfaces of both, the mixture of those two transparent 

 substances becomes opaque. Foam becomes opaque, not in virtue 

 of absorption, but of these repeated internal reflections. My assistant 



