CunnincHamM— Crystallization of Minerals in Igneous Rocks. 413 
It will, I am sure, be more satisfactory to show how I can 
obtain a safe lower limit for the latent heat of quartz (Plate XXIII.) 
from the results of other more experienced workers. Frémy in his. 
“Encyclopédie Chimique,”’ states that quartz is dissolved in hydro- 
fluoric acid with the evolution of no heat. Amorphous silica, on 
the other hand, when added to fuming aqueous hydrofluoric acid, 
evolves so much heat that the liquid boils. JI have therefore 
assumed that a weighed quantity of silica is added to the least 
possible relative proportion of the strongest possible hydrofluoric 
acid at 20°, and that the heat evolved is only just able to warm up. 
the liquid to 100° C. 
The reactions which take place are :— 
36% HE 
Se SSS SS 
3{Si02 + 4HF + 7-9H20} = 3{SiF, + 2H20 + 7°9H20} ... + Q cal. 
and 3SiFs + 2H20 =2(2HF, SiFi) + S8i02 eich acd Ongar, 
(Mean of Hammer] & Guntz.) 
Combining these two we obtain for the final result of the 
reaction :— 
3{SiO2 + 4HF + 7-9H.0} = 2(2HF, SiFy) + SiO. + 27-7H20 + (Q +48) cal. 
Then, according to Frémy’s statement, this quantity of heat 
(Q+ 48) is more than sufficient to raise the products of the reaction 
to the boiling-point, let us assume from 20° to 100° C. We do 
not know the specific heat of these products, but they may be 
fairly safely deduced from Dulong and Petit’s Law, according to 
the following table, the known data being supplied from Landolt 
und Bornstein’s ‘“‘ Physikalisch-Chemische Tabelien ” (p. 339) :— 
Substance. Mol. Weight. Specific Heat. ils li¥o 8 Salil 
(= const.) 
| 
ne : : 128-0 0-055 7°04 
Bie.) ) s 81-0 0-082 6-64 
NOU 4s 36-4 0-194 7-06 
1200 eae 20-0 0-346 (2) (6-92) 
OMe os Wnics-G | 0-132 22°49 
Sian. ib 104-0 0-216 (?) (22-42) 
