66o THE POPULAR SCIENCE MONTHLY. 



should say, perhaps, that the question of glacier motion presents 

 difficulties not yet wholly explained. There can be no douht, 

 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 pass- 

 ing into it from an electric lamp. Ice usually melts by con- 

 ducted heat, which reaches first the outside layers. But if we em- 

 ploy 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 pro- 

 ject upon the screen. We see that the melting begins at certain 

 points, which develop a crystallized 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 formation of apparently empty space. He care- 

 fully melted under water such a piece of ice, and found that 

 when the cavity was melted out there was no escape of air, prov- 

 ing 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 impor- 

 tance 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. 



I must not dwell further upon isolated questions, however 

 interesting, but will pass on at once to our main subject, which 

 may be divided into three distinct parts, relating namely to 

 heat, especially dark radiation, sound, and the behavior of 

 small particles, such as compose dust, whether of living or dead 

 matter. 



The earlier publications of Tyndall on the subject of heat 

 are for the most part embodied in his work entitled Heat as a 

 Mode of Motion. This book has fascinated many readers. I 

 could name more than one now distinguished physicist who 

 drew his first scientific nutriment from it. At the time of its 

 appearance the law of the equivalence of heat and work was 

 quite recently established by the labors of Mayer and Joule, and 

 had taken firm hold of the minds of scientific men ; and a great 

 part of Tyndall's book may be considered to be inspired by and 

 founded upon this first law of thermodynamics. At the time of 

 publication of Joule's labors, however, there seems to have been 

 a considerable body of hostile opinion, favorable to the now ob- 

 solete notion that heat is a distinct entity called caloric. Look- 

 ing back, it is a little difficult to find out who were responsible 

 for this reception of the theory of caloric. Perhaps it was rather 



