1894] on the Scientific Work of Tyndall 217 



vibration the resolution into drops takes place more rapidly, so that 

 the place of resolution travels up closer to the nozzle. 



Tyndail's observation was that the carrying down of air required 

 a jet already resolved into drops when it strikes the liquid. I hope 

 to be able to show you the experiment by projection upon the screen. 

 At the present moment the jet is striking the water in the tank pre- 

 vious to resolution into drops, and is therefore carrying down no air. 

 If I operate on the nozzle with a vibrating tuning-fork, the resolution 

 occurs earlier, and the drops now carry down with them a consider- 

 able quantity of air. 



Among the earlier of Tyndail's papers are some relating to ice, a 

 subject which attracted him much, probably from his mountaineering 

 experiences. About the time of which I am speaking Faraday made 

 interesting observations upon a peculiar behaviour of ice, afterwards 

 called by the name of regelation. He found that if two pieces of ice 

 were brought into contact they stuck or froze together. The pressure 

 required to produce this effect need not be more than exceedingly 

 small. Tyndall found that if fragments of ice are squeezed they pack 

 themselves into a continuous mass. We have here some small ice in 

 a mould, where it can be subjected to a powerful squeeze. The ice 

 under this operation will be regelated, and a mass obtained which 

 may appear almost transparent, and as if it had never been fractured 

 at all. The flow of glaciers has been attributed to this action, the 

 fractures which the stresses produce being mended again by regela- 

 tion. I should say, perhaps, that the question of glacier motion 

 presents difficulties not yet wholly exj^lained. There can be no 

 doubt, however, that regelation plays an important part. 



Another question treated by Tyndall is the manner in which ice 

 first begins to melt under the action of a beam of light passing into it 

 from an electric lamp. Ice usually melts by conducted heat, which 

 reaches first the outside layers. But if we employ a beam from an 

 electric lamp, the heat will reach the ice not only outside but 

 internally, and the melting will begin at certain points in the 

 interior. Here we have a slab of ice which we project upon the 

 screen. We see that the melting begins at certain points, which 

 develop a crystallised appearance resembling flowers. They are 

 points in the interior of the ice, not upon the surface. Tyndall found 

 that when the ice gives way at these internal points there is a forma- 

 tion of apparently empty space. He carefully melted under water 

 such a piece of ice, and found that when the cavity was melted out 

 there was no escape of air, proving that the cavity was really 

 vacuous. 



Various speculations have been made as to the cause of this 

 internal melting at definite points, but here again I am not sure if 

 the difficulty has been altogether removed. One point of importance 

 brought out by Tyndall relates to the plane of the flowers. It is 

 parallel to the direction in which the ice originally froze, that is, 

 parallel to the original surface of the water from which it was formed. 



Q 



