CONTROL OF THE BRILLIANCY OF LIGHT. 95 



the course of the phenomenon seems to be directed by intrinsic brilliancy, there can be 

 very little doubt that, for the same colour, or for white light, the law is rigidly observed, 

 a brighter illumination or a greater quantity of light producing an increased effect, a 

 lesser illumination or a feebler light producing a diminished effect. 



379. It seems to be substantiated by geological facts, that in former ages the rapidity 

 of vegetable growth was far greater than it is now ; that certain plants, which with 

 us attain only to an insignificant size, in those times reached a very great magnitude. 

 So ferns, which in our latitudes are now of an insignificant growth, in the same places 

 were formerly evolved into trees. The general character of vegetation, also, in given 

 latitudes, points to a former period of greater luxuriance a period during which trop- 

 ical trees could grow in the temperate zone. Geologists have already concluded, from 

 these and a variety of other observations, that the surface temperature has undergone 

 a diminution. 



380. But there are many conditions which have to be fulfilled before so marked a 

 difference could have taken place. The rate of vegetable growth depends on many 

 things : on the amount of aqueous vapour and carbonic acid in the air, on the mean tem- 

 perature of the surface, on the brilliancy of the incident light, &c. There are experi- 

 ments which seem to show that the constitution of the atmosphere is by no means so 

 favourable as it might be ; a greater quantity of carbonic acid in it is attended by a 

 greater rapidity of growth, and since those times of which we are speaking, great chan- 

 ges have taken place in this respect. Of the carbonic acid of those eras much is now 

 removed, and shut up in the earth in those great deposites of anthracite and bituminous 

 coal which occur on an extensive scale in so many parts of the world. Large quanti- 

 ties, also, unquestionably derived from the same source, now form an integral constitu- 

 ent of great coral reefs and limestone rocks, some forming mountain ranges, and some 

 in the sea. It can admit of little doubt that, since those times, the total quantity of 

 organized carbon existing on the earth's surface, and constituting the parts of plants 

 and animals conjointly, has been on the increase an increase attended by a diminu- 

 tion of the quantity unorganized in the atmosphere. M. DUMAS has well observed, 

 that the original atmosphere has become divided into three parts : one which still, in 

 a modified form, envelops the earth on all sides, constituting its present atmosphere ; a 

 second, represented by the aggregate of vegetables and animals now existing on the 

 earth's surface for plants and animals are nothing but condensed air ; a third, envel- 

 oped in a fossil state in the bowels of the earth. And this tri-partition has been main- 

 ly effected by the agency of the sun. To restore things to their primordial condition, 

 all these must be mingled together in the gaseous form, and the forces that have been 

 derived from the solar rays restored back to that luminary again. 



381. But an excess or a diminution of carbonic acid in the air, provided the varia- 

 tion is within certain limits, would not exert an exclusive control over the production 

 of vegetable organized molecules. Variations of temperature, as common observation 

 shows, exert a very great effect ; the periodicity in the seasons and processes of horti- 

 culture are sufficient to prove this. So well, indeed, has this been understood, that ge- 

 ologists, from considerations on the decline of vegetable growth, have drawn the doc- 



