300 Mr. A. S. Davis on a Theory of Nebula and Comets. 



other flat ring similarly marked. Twenty grains of the double 

 salt heated in a porcelain crucible became 7\, thus giving 62^ 

 per cent, of water of crystallization. One ounce of this salt with 

 half an ounce of water was boiled and filtered into two tubes : 

 when cold, the tubes were reduced to 0°, when, after some time, 

 the cup-shaped cavity at the bottom of the tube became lined 

 with an anhydrous powder, from which grew small, sharp, aci- 

 cular crystals. After this the capillary curve of liquid at the 

 surface became solid and shot down acicular crystals. Suddenly 

 the sulphate of soda separated at the surface and shot downwards 

 in the well-known crystalline lines. 



But amidst all these variations the main conclusion from this 

 inquiry is left undisturbed, namely, that highly supersaturated 

 solutions, chiefly (but not entirely) of double salts, when preserved 

 from the action of nuclei and reduced to low temperatures, form 

 compounds of various degrees of hydration, which can exist only 

 at such temperatures and in close vessels chemically clean. 



Highgate, N., 

 September 9, 1870. 



XXXVII. Addendum to a Theory of Nebula and Comets. By A. 

 S. Davis, B.A. y Mathematical Master, Leeds Grammar School. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



THERE is one point in the theory of nebulae and comets, 

 published in the June Number of your Magazine, which 

 may appear unsatisfactory. In that theory I suppose that ne- 

 bulse are formed by masses of gas coming into collision and 

 forming, where they may become mixed, a chemical union, with 

 evolution of light and heat. 



It may be objected that two masses of gas on becoming mixed 

 when in a cold and highly rarefied condition do not generally 

 combine, or do not at least combine with an energy sufficient to 

 produce light. To this objection I would answer, that though 

 the masses of gas on approaching each other are in a cold and 

 highly rarefied state, yet when they meet, the parts which come 

 into collision become so much condensed and heated that they 

 are put into a condition very favourable to their chemical union. 

 This I will endeavour to show. 



When two masses of gas come into collision, the portions which 

 first meet will act only to a very small extent as a cushion or 

 buffer to the remaining portions ; their velocity is so great that 

 the increased pressure produced by the condensation at the col- 

 liding surfaces has not time to transmit itself into the interior por- 



