72 BRIDGMAN. 



NH4SCN. This substance was Kahlhaum's purest, "zur Analyse," 

 used without further purification, except heating to 100° for several 

 hours in a vacuum, to remove all traces of moisture. Measurements 

 were made with three different fillings of the apparatus. The first 

 two were at the higher pressures, and for these the dry NH4SCN 

 powder was hammered into a compact mass in a steel shell, open at 

 both ends and with perforations in the sides. Pressure was trans- 

 mitted directly by kerosene. The third filling was for the run at low 

 pressures, and for this the dry powder was hammered into the in- 

 verted nickel steel shell, and pressure transmitted to it by mercury. 

 The difference in the pressure transmitting medium caused no per- 

 ceptible difference in the behavior. The quantities used were about 

 34 gm. for the first two fillings, and 21 gm. for the third. 



A transition point was first found at about 2200 kgm. at room 

 temperature. NH4SCX was known to be dimorphous, but the tran- 

 sition at atmospheric pressure is about 90°, so it seemed at first that 

 there was here a third new modification. On determining the transi- 

 tion at other temperatures, however, it appeared that this was not a 

 new form, but that the transition line runs from high to low tempera- 

 tures with increasing pressure; that is, the high temperature form has 

 the smaller volume. This somewhat unusual behavior does not seem 

 to have been noticed ; at any event this fact has not found its way to 

 tables of crystalline properties, nor in Gossner,* who has published the 

 most extensive investigation on this substance. Four points were 

 found with the first filling of the apparatus, from 0° to 67°. The 

 temperature was then raised to 200° in the search for other modifica- 

 tions. At about 4500 kgm., an irreversible change took place, with 

 an increase of volume of about 0.04 cm' gm. No other transition 

 could be found on increasing pressure to 12000 kgm., and then no 

 transition at all on releasing pressure from 12000, although the 

 melting curve ought to have been crossed. On opening the apparatus, 

 it was found that the irreversible transition was really a decomposition. 

 Very little gas was given off by the decomposed products, but the 

 smell was probably the worst that has ever been artificially produced. 

 The solid products of the decomposition were of a bright yellow color, 

 probably due to free sulfur, but no further analysis was attempted. 

 The products of the decomposition found their way through the whole 

 of the apparatus, and deposited themselves on the manganin coil, 

 short circuiting it, so that it had to be thrown away. The secontl 

 filling of the apparatus was made to redetermine one point at about 

 2200 kgm., the AT' value for which did not fall on a smooth curve 



