126 Proceedings of the Pioyal Society 
The paper concludes with some speculations as to the growth of 
ice crystals and the evaporation and condensation of aqueous 
vapour at water surfaces of various curvatures. 
Since making the experiments above described, the fog-producing 
powers of the products of highly heated chloride of magnesium 
have been tested, and are found to possess a much greater fog- 
producing power than any other substance experimented with. A 
few grains of this salt heated in an alcohol flame, in the unsatu- 
rated air of the cellar already referred to, gave rise to a fog many 
times denser than that produced by the sulphur or chloride of 
sodium, and remained hanging in the air of the cellar for more than 
six hours. When the experiment is made in the saturated air of 
the glass receiver, the fog rapidly grows so thick that in a few 
minutes it is impossible to see through more than 2 or 3 inches 
of it. The results are similar whether the salt is heated in a flame 
or in the platinum tube. This intense fog-producing power of 
highly heated chloride of magnesium would seem to be produced 
by highly concentrated hydrochloric acid, produced by the decom- 
position of the water and chloride of magnesium. 
[Added March 8th 1881.] 
It is found that the fog-producing power of the products of com- 
bustion of sulphur are greatly increased when these are mixed with 
other gases and vapours. For instance, a little ammonia — another 
of the products of combustion of our fires — when added to the 
sulphurous fumes, makes the fog many times more dense than that 
produced by the sulphur fumes alone. 
5. Note on Thermal Conductivity, and on the Effects of Tem- 
perature-Changes of Specific Heat and Conductivity on 
the Propagation of Plane Heat Waves. By Professor Tait. 
In the great majority, at least, of investigations (experimental 
or mathematical) connected with conduction of heat, it has been 
assumed that the known changes of specific heat of metals do not 
require to be taken into account. Thus Angstrom says, even in his 
paper on the Change of Conductivity with temperature ( Pogg . 118, 
1863) : — “Ha indess diese Yeranderungen, soweit man sie kennt, 
