366 Dr. T. Sterry Hunt's Lecture— 



should judge that this abundance of carbonic acid favoured a wonder- 

 ful development of vegetation, and, at the same time, the elimina- 

 tion of the carbon in the shape of coal, helped powerfully to purify 

 the air at that time. 



But there is another little question which comes up with regard to 

 the vegetation of these early times. We find, not only under the 

 tropics, but over the whole of the British Islands, and even within 

 the Arctic circle, a wonderful vegetation. Ferns and palms, such 

 as are now found growing only within the tropics, once grew within 

 the Arctic circle, and, indeed, very near the Poles. A very curious 

 and beautiful explanation of that has recently been presented by 

 the experiments of Dr. Tyndall, which I believe have been first 

 made public in this very Eoyal Institution. He observes with 

 regard to carbonic acid, and many other gases of that kind, that 

 their relations to radiant heat, and notably to obscure heat, were 

 such that a very small proportion — a few hundredths — of that car- 

 bonic acid diffused through the atmosphere would be quite sufficient 

 to prevent, almost entirely, the radiation of obscure heat from the 

 earth's surface ; so that an atmosphere constituted, as I have shown, 

 from chemical grounds, the atmosphere of these early times must 

 have been constituted, would permit the solar heat to pass through 

 our atmosphere, but would prevent its escape by radiation after it 

 had once heated the surface of the earth, and would thus immensely 

 augment the temperature of the lower strata of the atmosphere, 

 producing an effect precisely as if we had covered the whole earth 

 with an immense dome of glass, — had transformed it into a great 

 Orchid-house, — and had thus established, from the equator to the 

 poles, a moist, warm, equable climate, which would permit, even 

 within the limits of the polar circle, a luxuriant vegetation. 



This wonderful explanation of one of the most obscure problems 

 of geology, comes from the investigation of the relation of different 

 gases to radiant heat. 



I might go on still farther — (the subject tempts me) — and speak 

 of another curious class of phenomena that went on at the earth's 

 surface in these earlier times, and with regard to which the atmo- 

 sphere of that period played a very conspicuous part — the formation 

 of Magnesian limestone, or Dolomite. You all know how over great 

 parts of England the Mesozoic rocks are made up in great pro- 

 portions of carbonate of lime combined with magnesia. Wherever 

 you have gypsum beds you have a large quantity of this dolomite. 

 The formation of this substance has been a very obscure problem. 

 I have found, however, that by certain reactions in which carbonate 

 of lime intervened with the salts of the sea — reactions hitherto 

 unsuspected — it was possible to explain the formation not only of 

 gypsum, but of dolomite, but with one exception : I found that the 

 carbonic acid, which was an indispensible condition in the success of 

 that experiment, was constantly going off by difi'usion. My ex- 

 periments were constantly interrupted by the spontaneous evolution 

 of the gas. I remembered, however, that we must admit tliat in the 

 earlier times we had an atmosphere with several hundredths of 



