96 Mr. C. Tomlinson's Experimental Examination 



but much higher, partly at the top ; sometimes white spots will 

 appear in motion, particularly in the first part of the change — 

 that denotes storm. 



" Experience has proved this instrument to be highly useful in 

 the prognostics of the changes that take place in the atmosphere, 

 if properly and correctly made. 



"The above instrument has been observed to indicate rain 

 forty- eight hours before it took place." 



I have been in the habit of observing this instrument for some 

 years past, and had long ceased to regard it as of any value as a 

 weather-indicator, but supposed the various phenomena to be 

 brought about by the action of heat and light. While engaged, 

 however, in investigating the phenomena which accompany the 

 motion of camphor towards the light*, I was led to attend more 

 minutely to the storm-glass, and arrived at the conclusion that 

 heat is the only agent concerned, although the phenomena may 

 be complicated somewhat by the composition of the mixture and 

 the repeated crystallizations and solutions of the ingredients. 

 For example, nitre, which is freely soluble in water, is insoluble 

 in alcohol, the addition of which throws down a portion of the 

 salt from its aqueous solution; camphor, on the other hand, is 

 freely soluble in alcohol, and very sparingly so in water, so that 

 the addition of water to camphorated spirit precipitates camphor. 

 The mixture is so composed that a portion of the solid is always 

 precipitated ; and the effect of a rise of temperature is to take 

 up more of the solid, and of a fall in temperature to precipitate 

 it. As the liquid is in a state of saturation at a certain tempe- 

 rature, the top crust and the lower deposit come into contact as 

 that temperature is approached ; but this seldom happens, since 

 the crystallization of the solid renders sensible an amount of 

 latent heat, which again increases the solvent power of the men- 

 struum independently of the external temperature. In fine 

 weather also, especially if the glass have access to the external 

 air by means of a small hole or otherwise, there is a strong ten- 

 dency for crystallization to take place at the surface where the 

 air, the crystals, and the sides of the glass meet. Adhesion 

 is thus promoted, so much so that the surface of the crust may 

 become dry and throw off vapour of camphor, which forms a cry- 

 stalline deposit on the dry portion of the glass above the liquid 

 on the side nearest the light, that is, on the coldest side. This 

 upper crust is the principal seat of manufacture of the beautiful 

 feathers with transparent quills which may be often seen heaped 

 up on the lower deposit. As the feathers form above, they push 

 the feathers already formed away from their contact with the 



* See Philosophical Magazine for November 1862. 



