72 



ing that their heating effect is usually greater than those of the 

 visible spectrum. 



The laws of growth or vitalit}' are the laws of physics and mechan- 

 ics and chemistry as applied to living cells. The changes that go 

 on slowly in the plant are not the same as would go on rapidly in 

 large masses of the same chemicals Avhen treated as in the ordinary 

 chemical laboratory. In the plant small masses are confined within 

 the transparent walls of the cells until that subtile influence which 

 Ave call radiation can do its work in bringing about new combinations 

 of the atoms. It matters not whether we consider the radiation as 

 an orthogonal vibration, as in light, or a promiscuous interpenetration 

 of the molecules, as in heat, or a radial vibration, as in the waves 

 of sound ; Avhatever view we take of it, or whatever the details may 

 be, even if it be a rythmic breaking up and re-formation of the mole- 

 cules, the general characteristic of radiation is an extremely rapid 

 motion along the molecules and atoms of matter. Therefore, by 

 radiation we understand energy or momentum in the minute atoms 

 that go to make up the molecules and the masses that we deal with ; 

 this implies that work is done by one atom upon its neighbor, which 

 work, according to its style, we call light, heat, evaporation, etc. 

 Assimilation and transpiration are among the forms of work in the 

 growth of the plant that are due to the molecular energy contained 

 in sunshine, and it is essential to progress in agriculture that there 

 be kept a continuous register of the intensity and nature of the solar 

 radiations that reach the plant. But this is a difficult problem, whose 

 satisfactory solution has not yet been attained, although the work 

 of Violle, Bunsen and Roscoe, Marie Davy, Marchand, Langley, Roav- 

 land, Hutchins, and many others have marked out the methods which 

 seem most promising. 



ANNUAL DISTRIBUTION OF SUNSHINE. 



Humboldt (1815), in his chapter on ''Climate," after comparing 

 the climates and fruits of Europe, says: 



These comparisons demonstrate how important is the -diversity of 

 the distribution of heat throughout the different seasons of the year 

 for the same mean annual temperature, as far as concerns vegetation 

 and the culture of the fields and orchards, and as well as regards our 

 own well-being as a consequence of these conditions. 



The lines which I call isochimenal and isotheral (lines of equal tem- 

 perature for winter and summer) are not parallel to the isothermal 

 lines (lines of equal annual temperature) in those countries where — 

 notwithstanding the myrtle grows wild in its natural state, and where 

 no snow falls during the winter — the temperature of summer and fall 

 scarcely suffices to bring apples to full maturity. If to give a potable 

 wine tile vine shuns the islands and nearly all sea coasts, even those 

 of the west, the cause is not only in the moderate heat of summer upon 

 the seashore, a circumstance which is shown by thermometers exposed 



