160 BRIDGMAN. 



only others. (NH4)2KP04 has a transition when anhydrous, but not 

 when hydrous. It is to be remembered, however, that only one of 

 these substances belongs to the class whose hydrates have a transition 

 point at atmospheric pressure. The investigation of other substances 

 with known transition points of the hydrates offers a field of consider- 

 able interest, but the phenomena are more complicated than those of 

 the simple polymorphic transitions considered here. 



The phenomena with FeS047H20 are as follows. It showed two 

 transition points. At 10,000 kgm. there is a transition with fairly 

 large decrease of volume somewhere near 80°; that is, this transition 

 is of the ice type. At 85° there is no transition between 10,000 and 

 2000, but on cooling at 2000 there is a much smaller reverse transi- 

 tion near 35°. This point is doubtless connected with the known 

 dehydration points at atmospheric pressure, and the other point at 

 10,000 is probably of the same nature, rather than a true polymorphic 

 transition. It is evident that the dehydration diagram of this sub- 

 stance must be rather complicated. I made no further attempt to 

 study it; there are several serious difficulties in the way of a complete 

 investigation. 



Double Salts. — (NH4)2KP04, anhydrous,* and with 4H2O; 

 KNH4SO42H2O; ZnK2(S04)26H20, anhydrous* and with 6H2O. 



All of these runs were made at 20° and 100°, except K2Zn(S04)2. 

 The first substance has been described under the last heading, and 

 K2Zn (804)2 has been described in a separate section. It is likely that 

 both of these transitions are really decompositions to the simple 

 salts, rather than a true polymorphic change. 



An attempt to make KNH4SO42H2O anhydrous by heating for a 

 number of hours at 150° in vacuum was without success. 



Substances with Unstable Forms. — Benzophenone, has four 

 modifications including a monoclinic and a rhombic form; Para-nitro 

 phenol; Acetamide*, trigonal; Menthol; Acetophenone ; Mono- 

 chloracetic Acid, has several unstable forms; Propionic Acid; SbIs, 

 trigonal; Sulfur*, has a stable rhombic and monoclinic form, and 

 numerous unstable forms; Phosphorus,* cubic, trigonal; Antimony, 

 trigonal; Selenium, two monoclinic and a trigonal form; Arsenic, 

 cubic, trigonal; Iodine, rhombic, monoclinic. 



The first seven of these substances have already been commented 

 on, either in this or in earlier papers; the melting curves of several 

 of them have been determined. Phosphorus ^ has been made the 

 subject of a special paper. Antimony, besides being very near phos- 

 phorus in the periodic table, forms a number of stable and unstable 



