Dr. A. Waller's Microscopic Observations on Hail. 103 



diluted state, and to dispense with the dipping in water alto- 

 gether. 



The diluting is a very much simpler and more oeconomical 

 method, and it is evidently a more certain one. 



If the gallo-nitrate be diluted not more than ten times, it 

 imparts a high degree of sensibility, but in that case the paper 

 must be used immediately. It may be diluted as much as 

 forty times, and in favourable circumstances will still give an 

 intense picture. In diluting to that extent the paper becomes 

 less sensitive, but it is less subject to the spontaneous change, 

 and preserves its useful properties much longer. 



To develope an impression, the gallo-nitrate ought to be of 

 the full strength, particularly in gallic acid, upon which the 

 depth and intensity of the picture chiefly depend, and which 

 in a saturated solution will be present in greater or less quan- 

 tity, according to the temperature of the air, by which the 

 general chemical action is also affected. 

 London, July 7, 1846. 



XX. Microscopic Observations on Hail. 

 By A. Waller, M.D.* 



IN a paper read before the Royal Society, June 18, 1846, 

 -^ I have presented some observations on the microscopic 

 examination of globules of water, as they exist in fumes of 

 steam, fogs, clouds, and mists. I found that they consist of 

 particles of water perfectly spherical, without the least ap- 

 pearance of a vesicular structure, as is generally taught. I 

 now proceed to examine, in a similar manner, water in some 

 of the other forms in which it presents itself in meteorology, 

 and commence with hail. 



Some confusion seems to exist between different authors 

 respecting the distinction between hail and sleet, which it is 

 very desirable to avoid. Kaemtz, in his Manual of Meteoro- 

 logy, states that "very small hailstones are termed sleet." 

 Pouillet, Elemens de Physique, who is generally very precise 

 in his terms, gives the following definition : " Le gresil [sleet], 

 which we have the opportunity of observing in our climates 

 almost every year during the months of March and April, 

 has doubtless an origin similar to that of snow. It is also 

 congealed water, or rather small needles of ice pressed and 

 interlaced together, forming a kind of ball of rather a compact 

 nature, and sometimes surrounded with a real coating of 

 transparent ice." In one dictionary I find sleet termed "a 

 fall of rain and snow together," and in another, " a kind of 



* Communicated by the Author. 



