22 ROYAL SOCIETY OF CANADA 



whole or in part, to the same cause; but if so, the corresponding in- 

 clusions of air must necessarily be much more attenuated and more widely 

 diffused than in the former cases. We can only conjecture as to the 

 manner in which these minute air-tubes and blisters are formed. 



" As no one can ever actually see the extremely minute water- 

 particles rush together and form themselves into snow-crystals, the 

 material and the manner in which the molecules of water are joined to 

 form snow-crystals is largel}' a matter of speculation. While it is true 

 that the snow-crystals form within the clouds, it does not therefore 

 follow tliat they are formed from the coarse particles of which the clouds 

 are composed in cold weather. 



" We have good grounds for assuming that the true snow-crystals 

 are formed directly from the minute invisible atoms or molecules of 

 water in the air, and not from the coarse particles in the clouds, as it is 

 unlikely that these coarse particles could unite into snow-crystals in so 

 perfect a manner as to leave no trace of their union even when examined 

 under powerful microscopes."' 



Structure of Solid Ice. — The method of studying the crystalline 

 structure of a solid block of ice was given us by Tyndall. Slabs or plates 

 of ice, about half an inch in thickness, were cut parallel to the freezing- 

 planes, and rendered flat by levelling and reducing by means of a warm 

 and smooth metal plate. The planes of freezing were found by observing 

 the direction of the bubbles in the ice, which were either arranged in 

 striae at right angles to the surface, or collected in groups parallel to the 

 surface of the water. A beam of sunlight was then focussed by a lens 

 on points in the interior of the slab. The position of the focus was first 

 found in the air, the lens was then screened and the ice placed in posi- 

 tion, the screen was removed and the effect watched through an ordinary 

 pocket-lens. By this means the path of the heat-ray was instantly stu- 

 died by a great number of little luminous points resembling shining air- 

 bubbles. When the beam was sent through the edge of the plate, the 

 path of the beam could be traced by these brilliant spots. 



In lake-ice the planes of freezing are easily recognized by the strati- 

 fied appearance, which the distribution of the air-bubbles gives to the 

 substance. When surfaces perpendicular to the planes of freezing were 

 examined by a lens, after exposure to the light, they were found to be 

 cut by innumerable small parallel fissures, with here and there minute 

 spurs shooting from them, which gave the fissures, in some cases, a 

 feathery appearance. When ordinary light from the window was allowed 

 to fall on the ice at a suitable incidence, the interior of the mass was- 

 found filled with little flower-shaped figures. Each flower was found to 

 consist of six petals, and at its centre was a bright spot which shone with 



