136 Dr. Tyndall on Molecular Influences. 



exposed to the sun becomes burning hot as the hour of maximum 

 temperature approaches; but, after this is passed, the heat is 

 yielded up with proportionate facility. Hence a maximum and 

 minimum thermometer must, in the latter case, mark a far wider 

 range of temperature during the twenty-four hours than in the 

 former. This agrees with observation. In Sahara, to use the 

 words of Mrs. Somerville, during "the glare of noon the air 

 quivers with the heat reflected from the red sand, and in the 

 night it is chilled under a clear sky sparkling with its host of 

 stars *." Were gypsum, however, the prevailing mineral, it is 

 a priori certain that this could not be the case to anything like 

 its present extent. 



The following experiments furnish some notion of the trans- 

 missive power of a few other organic structures ; cubes of the 

 substances were examined in the usual manner. 



Tooth of Walrus 16 



Tusk of East Indian Elephant . 17 



Whalebone 9 



Rhinoceroses-horn 9 



Cow^s-horn 9 



Considering the density and elasticity of ivory, we might be 

 disposed to attribute to it a comparatively high conductive power ; 

 but the experiment proves it to be a very bad conductor — much 

 inferior, indeed, to wood in the direction of the fibre. Doubtless 

 this conduces to the animal's comfort. Exposed to the rays of a 

 tropical sun, if these huge bony masses were capable of assuming 

 a high temperature during the day and losing it again at night, 

 it must be a source of the greatest inconvenience to the animal, 

 as at present constituted. The horns of the Rhinoceros and Cow, 

 however, still more strikingly exemplify that fitness of parts 

 which is perpetually presented to the student of natural science. 

 In the latter case especially, the mass of horn in close contact 

 with the skull, and therefore capable of transmitting heat di- 

 rectly to the animal's brain, must be attended with very un- 

 pleasant consequences, if horn were a good conductor. Given 

 such a constitution, the substance fixed upon by our own en- 

 lightened intellect to furnish the animal with such weapons of 

 defence, would be just such as nature has chosen. 



As a general rule, sudden changes of temperature are preju- 

 dicial to animal and vegetable health; the substances used in 

 the construction of organic tissues are exactly such as arc best 

 calculated to resist those changes. Coal enters largely into the 

 composition of such tissues, and it is an exceedingly bad con- 



* Physical Geography, vol. i. p. 147. 



