of' Snow around Plants. 247 



do not require the aid of the thermo-multiplicator, and which 

 bring out facts which are sometimes identical, and sometimes 

 diametrically opposed to those which have been pointed out by 

 M. Fusinieri. 



Having filled, and more than filled, a cylindrical vessel with 

 fine and newly-fallen snow, I removed all above its rim, by 

 means of a wooden ruler, so that its surface was a very uniform 

 plane of snow. I then turned this plane vertically, and upon 

 it I caused the rays of an argand-lamp to fall, after having sus- 

 pended before its central portion, and quite near to the surface 

 of the snow, a small disk of thin pasteboard, having both its 

 sides thoroughly covered with lamp-black. The rays of the 

 lamp, of course, darted partly on the disk and partly on the 

 snow. In a very short time the plain surface was hollowed out 

 beneath the disk, and at the end of a quarter of an hour this 

 cavity was not less than three or four lines deep at its centre. 



I next placed the apparatus in the same circumstances it was 

 at the commencement of the previous experiment, with the dif- 

 ference of substituting the copper plate at 400° of temperature 

 for the lamp. The phenomena then presented themselves quite in 

 theinverse way, that is to say,themeltingor corrosion of the snow 

 was more abundant, where the direct rays impinged, than in the 

 part situated opposite the disk, so that a protuberance, instead 

 of a hollow, was very soon formed in its centre. Hence it fol- 

 lows, that a certain energy of the incident heat is not sufficient 

 alone, to produce a greater action upon that part of the surface 

 which is shaded by the disk ; there is, moreover, necessary, that 

 peculiar quality of calorific radiation which is analogous to the 

 solar heat, and which, like it, is ordinarily accompanied with 

 luminous radiation, but which does not require it as a necessary 

 concomitant.* 



If the reasoning, which we have connected with the experi- 

 ment in which the grey paper was interposed before the white 

 painted thermo- electrical pile, is accurately understood, the ex- 

 planation of these differences in the melting will not present 

 any difficulty. 



In the former case, the heated pasteboard darted towards the 

 vessel, rays which were much more absorbable than the direct 

 * See Note 2, p. 255. 



