CHAP, iv.] APPLICATIONS OF THE LAWS OF HEAT. 383 



water are exposed to the clear firmament. The water is a powerful 

 radiant, and sends off its heat copiously into space. The heat thus 

 lost cannot be supplied from the earth, this source being cut off by 

 the non-conducting straw. Before sunrise a cake of ice is formed in 

 each vessel. This is the explanation of Wells, and it is, no doubt, 

 the true one. I think, however, it needs supplementing. It appears 

 from the descriptions, that the conditions most suitable for the forma- 

 tion of ice is not only a clear air but a dry air. The nights, says 

 Sir Robert Baker, most favourable for the production of ice are those 

 which are clearest and most serene, and in which very little dew 

 appears after midnight. The italicized phrase is very significant. To 

 produce the ice in abundance the atmosphere must not only be clear, 

 but it must be comparatively free from aqueous vapour. When the 

 straw on which the pans were laid became wet, it was always changed 

 for dry straw ; and the reason Wells assigned for this was, that the 

 straw, by being wetted, was rendered more compact and efficient 

 as a conductor. This may have been the case, but it is also certain 

 that the vapour rising from the wet straw and overspreading the pans 

 like a screen would check the chill and retard the congelation." 



V. ARTIFICIAL MANUFACTURE OF ICE. 



Ice is very largely used in these days in all civilised countries, 

 as it serves not only for cooling all sorts of drinks in summer for 

 making ices, creams, &c. but is used also in medicine and surgery in 

 the treatment of certain diseases and in dressing wounds. Its con- 

 sumption in Europe and America is considerable. It is obtained in 

 blocks from Eussia, Sweden, and Norway, and from the surface of the 

 lakes in Canada, whence it is carried by sea to the southern countries. 

 To transport these blocks without exposing them to melting by the 

 milder temperature of their destinations, they are arranged in layers 

 in boxes, which are surrounded and separated by sawdust. The 

 slight conductivity of this material is sufficient to protect the ice 

 during the voyage. On its arrival it is kept in ice-houses, from 

 which it is taken when required. 



But it has been attempted to make it on the spot, and at the 

 moment it is wanted. The apparatus invented for this purpose are 



