ADDRESS. 



liii 



The light and heat which are emitted from the sun reach the earth without 

 great diminution by the absorptive action of the atmosphere ; but the waste 

 of heat from the surface of our planet through radiation into space is pre- 

 vented, or rather lessened by this same atmosphere. Many transparent 

 bodies admit freely heat-rays derived from a source of high temperature, but 

 stop the rays which emanate from bodies only slightly warmed. The atmo- 

 sphere possesses this quality in a remarkable degree, and owes it to the pre- 

 sence of diffused water and vapour ; a fact which Dr. Tyndall has placed in 

 the clear light of complete and varied experiment *. The application of this 

 truth to the history of the earth and of the other planets is obvious. The 

 vaporous atmosphere acts like warm clothing to the earth. Hy an augmented 

 quantity of vapour dissolved, and water suspended in the air, the waste of 

 surface-heat of the earth would be more impeded ; the soil, the water, and 

 the lower parts of the atmosphere woiild grow wanner; the climates would 

 be more equalized ; the general conditions more like what has been supposed 

 to be the state of land, sea, and air during the geological period of the Coal- 

 measures. 



Such an augmentation of the watery constituents in the atmosphere would 

 be a natural consequence of that greater flow of heat from the interior, which 

 by many geologists, mathematicians, and chemists is supposed to have hap- 

 pened in the earlier periods of the history of the earth. 



By the same considerations we may understand how the planet Mars, which 

 receives not half so much heat from the sun f as the earth does, may yet 

 enjoy, as in fact it seems to enjoy, nearly a similar climate, with snows alter- 

 nately gathering on one or the other of its poles, and spreading over large 

 spaces around, but not, apparently, beyond the latitude of 50° or 40°; the 

 equatorial band of 30° or 40° north or south being always free from snow- 

 masses bright enough and large enough to catch the eye of the observer. 

 Mars may therefore be inhabited, and we may see in the present state of 

 this inquiry reason to pause before refusing the probability of any life to 

 Jupiter and even more distant planets. 



The history of suns and planets is in truth the history of the effects of 

 light and heat manifested in them, or emanating from them. Nothing in 

 the universe escapes their influence ; no part of space is too distant to be 

 penetrated by their energy ; no kind of matter is able to resist their trans- 

 forming agency. Many if not all the special forces which act in the parti- 

 cles of matter are found to be reducible into the general form of heat ; as 

 this is convertible and practically is converted into proportionate measures of 

 special energy. Under this comprehensive idea of convertibility of force, 

 familiar to us now by the researches of Joule J, the reasonings of Grove § and 

 Helmholtz, and the theorems of Eankine |[, it has been attempted by Mayer, 

 Waterston, and Thomson^! to assign a cause for the maintenance of the heat- 

 giving power of the sun in the appulse of showers of aerolites and small 



in 1769, he allowed 33 millions. Such was the effect of that now supposed erroneous ex- 

 periment on the opinions of astronomers. (Epoques de la Nature.) 



* Proc. of Koy. Soc. 1861. The Rumford Medal was adjudged to Dr. Tyndall in 1864. 



t The proportion is about ^§r according to the received measure of the mean dis- 

 tance. 



J Phil. Mag. 1843; Reports of the British Association, 1845 ; Trans, of the Royal 

 Society, 1850. 



§ Grove on the Correlation of Phvsical Forces, 1846. 



|| Rankine, Trans, of the Royal Society of Edinburgh, 1850-51 ; Phil. Trans, 1854. 



*|[ Communication to the Royal Society of Edinburgh, 1 854. 



