In his British patent,[25] for desulphurizing sulphide ores, A. D. Carmichael states that a mixture of lead sulphide and calcium sulphate reacts “at dull red heat, say about 400 deg. C.,” forming lead sulphate and calcium sulphide, according to the equation:
PbS + CaSO4 = PbSO4 + CaS.
Judging from thermo-chemical data, this reaction does not seem probable. According to Roberts-Austen,[26] the heats of formation (in kilogram-calories) of the different compounds in this equation are as follows: PbS = 17.8; CaSO4 = 318.4; PbSO4 = 216.2; CaS = 92. Hence we have the algebraic sum:
-17.8 - 318.4 + 216.2 + 92 = -28.0 cal.
As the law of maximum work does not hold, experiment only can decide whether this decomposition takes place or not. The following experiments were made:
Experiment 1.—Coarsely crystalline and specially pure galena was ground to powder. Some gypsum was powdered, and then calcined. The powdered galena and calcined gypsum were mixed in molecular proportions (PbS + CaSO4), and heated for 1½ hours to 400 deg. C., in a stream of carbon dioxide in a platinum resistance furnace. The temperature was measured with a Le Chatelier pyrometer. The material was allowed to cool in a current of carbon dioxide.
The mixture showed no signs of reaction. Under the magnifying glass the bright cube-faces of galena could be clearly distinguished. If any reaction had taken place, in accordance with the equation given above, no bright faces of galena would have remained.
Experiment 2.—A similar mixture was slowly heated, also in the electric furnace, to 850 deg. C., in a stream of carbon dioxide, and was kept at this temperature for one hour.
It was observed that some galena sublimed without decomposition, being redeposited at the colder end of the porcelain boat (7 cm. long), in the form of small shining crystals. The residue was a mixture of dark particles of galena and white particles of gypsum, in which no evidence of any reaction was visible under the microscope. That galena sublimes markedly below its melting point has already been noted by Lodin.[27]
Experiment 3.—In order to determine whether the inverse reaction takes place, for which the heat of reaction is + 28.0 cal., the following equations are given: