264 NATURE [Fuly 22, 1886 
: Heat of “ee Heat of ans Compound Heat of formation Compound Heat of formation 
ae paras as Difference Solution Difference [H,Cl, Aq] 30315 [H,Cl, Aq] 30315 
[K, Cl] 105610 — — 4440 = [H, Br, Aq] 28380 [H,1, Aq] 13170 
[H,Cl,Aq] _ 39315 66295 ae s ae Sno 
[K, Br] 95310 —_— _—5080 +640 Difference 10935 Difference 26245 
[H, Br, Aq] 28380 66939 — _ K],Cl,Aq] IOLI7O [K,Cl, Aq] IOII7O 
sotiite 3 [K, Br, Aq] 90230 [K,1,Aq] 75040 
—635 +640 Difference 10940 Difference 26130 
[K, Br] 95310 fickas — 5080 = Now the reason of this is perfectly obvious in the light of the 
[H,Br,Aq] _ 28380 66930 = — laws of solution. Any variation from the above differences in 
[K,1] 80130 — —5I1IO +30 the heat of formation of the undissolved salt is at once counter- 
[H,1, Aq] 13170 66960 = = balanced by the heat of solution, which varies inversely. 
cll Thus :— 
Heat Heat . 
=30 +30 Compound of formation of eaten aor 
(H,Cl] 22000 17315 39315 
These relations obtain for the haloid salts of all the metals (H, Br] 8440 19940 28380 
for which data were available for comparison. The only excep- —o 
tion is AuCl, and AuBr,, the difference of heats of solution of Difference 13560 — 2625 10935 
these salts being too great according to the foregoing laws. They [Na,Cl] 7690 ese 96510 
are apparently proportional, however. [Na, Br] 3 ree Dea Bcz80 
There is another way of showing these laws and also of show- ? 577° os 55 
ing the conditions which determine the absolute amount of heat of = = 
solution, whether positive or negative. If we take the sum of the Difference Lye) yee) eo 
heats of formation of any salt and of water on the one hand, and 
on the other, instead of measuring the heat of solution directly, 
take the sum of the heats of formation of the oxide, of the acid 
and of neutralisation, we shall find that the heat of solution is 
the difference of these sums—positive when the latter sum is the 
greater, and negative when it is the less. This exhibits in a 
striking manner the relations of the various affinities to solution, 
and is very suggestive when we consider that the heat of solu- 
tion regularly increases with the heat of formation of [M,O, Aq], 
and when the heat of [MO]>[M,Cl,], decomposition of water 
takes place. Consider the following examples :— 
Compound Sontieanie Compound Bereon 
[Mg,OAq] 148960 [Mg,Cl,] 151010 
[2H,Cl, Aq] 78630 [H,O] 68360 
Neutr. 27690 
255280 219370 
219370 
Difference 35910 = Heat of solution. 
[Sr,O, Aq] 157780 [Sr,S, Oy] 330900 
[H2,8,0,,Aq] 210770 [H2,0] 68360 
Neutr. 30710 
399260 399260 
399260 
Difference o=Heat of solution. Salt insoluble. 
[k.,0, Aq] 164560 [K,,N,0¢] 242970 
[H,,N.,O,,Aq] 102190 [H,,0] 68360 
- Neutr. 27540 
294290 311330 
311330 
Difference — 17040 = Heat of solution. 
The above illustrate the cases of positive, negative, and zero 
heats of solution. These relations obtain with all salts, whether 
the oxide is soluble or not. The only discrepancy I found was 
in the case of silver chloride, which showed a slight negative 
heat of solution ; but as its affinity for O is excessively small, it 
is not surprising it should be an abnormal case. 
These laws of solution explain and are illustrated by many 
cases of constant differences in the heats of formation of similar 
compounds in water. Thusit has been pointed out in Muir and 
Wilson’s ‘‘ Thermo-Chemistry”’ that between the heats of for- 
mation of soluble chlorides, bromides, and iodides in water, there 
is a constant difference, no matter what the positive element is. 
For example, consider the following cases :— 
from N.W. 
and so on in other cases. Wo. DuRHAM 
Ice on the Moon’s Surface 
In May 1884 Mr. Peal, of Sibsagar, in Assam, who has 
studied the moon’s surface with great attention, sent me a paper 
in which he maintained views closely resembling those of Capt. 
Ericsson (NATURE, p. 248) on the glacial origin of the lunar 
craters. In my answer I suggested that it was difficult to admit 
the existence of ice on the moon’s surface, without a layer of 
water vapour over it, and that the telescope proves that if such 
vapour exists it is only in extraordinarily small quantities. It 
seems due to Mr. Peal, who was undoubtedly ignorant of Capt. 
Ericsson’s paper of 1869, to draw attention to the correspond- 
ence. Jam not sure whether the paper has been yet published. 
Cambridge, July 17 G. H. Darwin 
Luminous Clouds 
I AM not sure of the date, but believe it was in June 1885 
that I called attention in your journal to a strange effect of bright 
silvery lighted ciouds, which remained visible in the north-west 
sky after sunset until nearly 11 p.m. Several times this summer 
I have noted repetitions of these same curiously lighted cloud- 
forms, but have never seen such a wonderful display of this 
‘*after-sheen ” as that of this evening, July 12. 
The day from 11 a.m. until 6 p.m. had been wet, followed by 
a clear-up toward sundown, with a warm orange-coloured sunset 
near the horizon; above this, and extending nearly to the 
zenith, lay masses of brilliant and, one would almost say, self- 
illuminated cloud-ripples looking like an inverted sea of frosted 
silver or mother-of-pearl. 
There was a strongly-marked focus in the light above the 
place of the sun, but it extended far beyond that both north and 
west. The vapour forming these cloud-waves, and which re- 
ceived this intense white light, must, I think, have been at a 
great elevation, for though all the lower vapour near the horizon 
retained its usual orange glow long after sunset, there was never 
any indication of colour upon these clouds from the beginning 
of the effect, about 7.30 p.m., until it disappeared soon after 
10 p.m. The moon, which was in the southern part of the sky, 
looked quite warm in colour when contrasted with the almost 
bluish-white glare upon this vapour. Rost, C, LESLIE 
Moira Place, Southampton, July 12 
THE luminous night clouds seen here on the 22nd ult. 
(NATURE, July 1, p. 192) have recurred, with a very remarkable 
development on the night of the 8th inst. 
The sketches illustrate phases one hour apart from midnight 
to 2a.m. ; the last made solely by ‘‘cloud-light” in a window 
with northern aspect! The long luminous belt began to form at 
11.30 p.m., fading out at 2,.30a.m. It extended obliquely from 
N. 10° W. to N. 30° E. in the wind’s direction, which was light 
Temperature subsequently fell. . 
Treo: 
