Our knowledge of the temperature variation of the principal susceptibilities of paramagnetic crystals is as yet fragmentary. The principal susceptibilities of a number of paramagnetic crystals have been determined by Finke and by Rabi at room temperature but the first measurements on orientated crystals over any range of temperature were those of Foëx. He has published the principal susceptibilities of siderose (a mineral which is mainly ferrous carbonate but which contains appreciable quantities of the carbonates of manganese and other metals) over the range 87° to 400°K. and some measurements but not actual principal susceptibilities for manganese sulphate, MnSO
4
.4H
2
O. The writer measured the principal susceptibilities of cobalt ammonium sulphate and nickel sulphate, NiSO
4
. 7H
2
O over the temperature range 14°-290°K. and the writer and de Haas have published measurements on manganese ammonium sulphate crystals over the restricted range 14°-20° K. In addition Dupouy has repeated Foëx’s observations on siderose and has measured the principal susceptibilities of dialogite (a naturally occurring manganese carbonate) and oligist (Fe
2
O
3
), all over a small range of temperature above 0° C. Quite recently Bartlett has employed Rabi’s method to determine the principal susceptibilities of the following compounds, CoSO
4
. 7H
2
O, CoSO
4
. (NH
4
)
2
SO
4
. 6H
2
O, CoSO
4
. K
2
SO
4
. 6H
2
O, CuSO
4
. (NH
4
)
2
SO
4
. 6H
2
O CuSO
4
. K
2
SO
4
. 6H
2
O, NiSO
4
. (NH
4
)
2
SO
4
. 6H
2
O over the temperature range —45° to +55°C. In the writer’s opinion, however, Bartlett’s procedure is liable to several objections.
Phonon hydrodynamics and ultrahigh–room-temperature thermal conductivity in thin graphite
