252 SCIENCE OF GARDENING Part II. 



been placed, by means of two upright sticks, perpendicularly to a grass-plot, and at right 

 angles to the course of the air, a thermometer was laid upon the grass close to the lower 

 edge of the handkerchief, on its windward side. The thermometer thus situated was 

 several nights compared with another lying on the same grass-plot, but on a part of it 

 fully exposed to the sky. On two of these nights, the air being clear and calm, the grass 

 close to the handkerchief was found to be 4° warmer than the fully exposed grass. On 

 a third, the difference was 6°. An analogous fact is mentioned by Gersten, who says, 

 that a horizontal surface is more abundantly dewed, than one which is perpendicular to 

 the ground. 



1207. Heat from a covering of snow. The covering of snow, the same author ob- 

 serves, which countries in high latitudes enjoy during the winter, has been very 

 commonly thought to be beneficial to vegetable substances on the surface of the 

 earth, as far as their temperature is concerned, solely by protecting them from the cold 

 of the atmosphere. But were this supposition just, the advantage of the covering 

 would be greatly circumscribed ; since the upper parts of trees and of tall shrubs 

 are still exposed to the influence of the air. Another reason, however, is furnished 

 for its usefulness, by what has been said in this essay ; which is, that it prevents the 

 occurrence of the "cold, which bodies on the earth acquire, in addition to that of 

 the atmosphere, by the radiation of their heat to the heavens during still and clear 

 nights. The cause, indeed, of this additional cold, does not constantly operate; but 

 its presence, during only a few hours, might effectually destroy plants, which now 

 pass unhurt through the winter. Again, as things are, while low vegetable produc- 

 tions are prevented, by their covering of snow, from becoming colder than the atmo- 

 sphere in consequence of their own radiation, the parts of trees and tall shrubs, which 

 rise above the snow, are little affected by cold from this cause. For their outermost 

 twio-s, now that they are destitute of leaves, are much smaller than the thermometers 

 suspended by me in the air, which in this situation very seldom became more than 2° 

 colder than the atmosphere. The larger branches, too, which, if fully exposed to the sky, 

 would become colder than the extreme parts, are, in a great degree, sheltered by them ; 

 and, in the last place, the trunks are sheltered both by the smaller and the larger parts, 

 not to mention that the trunks must derive heat, by conduction through the roots, 

 from the earth kept warm by the snow. In a similar way is partly to be explained the 

 manner, in which a layer of earth or straw preserves vegetable matters in our own 

 fields, from the injurious effects of cold in winter. (Essay on Dew, &c. 1819.) 



1208. The nature of light is totally unknown : the light which proceeds from the sun 

 seems to be composed of three distinct substances. Scheel discovered that a glass mir- 

 ror held before the fire reflected the rays of light, but not the rays of caloric ; but when 

 a metallic mirror was placed in the same situation, both heat and light were reflected. 

 The mirror of glass became hot in a short time, but no change of temperature took place 

 on the metallic mirror. This experiment shows that the glass mirror absorbed the rays 

 of caloric, and reflected those of light ; while the metallic mirror, suffering no change of 

 temperature, reflected both. And if a plate glass be held before a burning body, the 

 rays of light are not sensibly interrupted, but the rays of caloric are intercepted ; for no 

 sensible heat is observed on the opposite side of the glass ; but when the glass has reached 

 a proper decree of temperature, the rays of caloric are transmitted with the same facility 

 as those of fight. And thus the rays of light and caloric may be separated. But the 

 curious experiments of Dr. Herschel have*clearly proved that the invisible rays which 

 are emitted by the sun, have the greatest heating power. In those experiments,^ the dif- 

 ferent colored rays were thrown on the bulb of a very delicate thermometer, and their heat- 

 ing power was observed. The heating power of the violet, green, and red rays were found 

 to be to each other as the following numbers: violet, 16-0; green, 22-4 ; red, 55*0. 

 The heating power of the most refrangible rays was least, and this power increases as 

 the refrangibility diminishes. The red ray, therefore, has the greatest heating power, 

 and the violet, which is the most refrangible, the least. The illuminating power, it has 

 been already observed, is greatest in the middle of the spectrum, and it diminishes to- 

 wards both 'extremities ; but the heating power, which is least at the violet end, increases 

 from that to the red extremity ; and when the thermometer was placed beyond the limit of 

 the red ray, it rose still higher than in the red ray, which has the greatest heating power 

 in the spectrum. The heating power of these invisible rays was greatest at the distance 

 of half an inch beyond the red ray, but it was sensible at the distance of one inch and a 



half. 



1209. The influence of the (liferent solar rays on vegetation has not yet been stu- 

 died ; but it is certain tha't the rays exercise an influence independent of the heat they 

 produce. Thus plants kept in darkness, but supplied with heat, air, and moisture, grow 

 for a short time, but they never gain their natural colors ; their leaves are white and 

 pale, and their juices watery and peculiarly saccharine : according to Knight they merely 



