SCIENTIFIC SUMMARY. 
275 
with absorptive spectra than samarskite. In addition to terbia there was 
found a yellow earth with an equivalent of about 90, and without an 
absorptive spectrum, which appeared to be a mixture of terbia and yttria. 
The Vapour Density of the Halogens . — The researches on this subject, 
carried on by Victor Meyer, in association with Carl Meyer, on the dissocia- 
tion of chlorine at a red heat, have opened the field to a great number of 
researches, the publication of which they withhold till all the difficulties 
incident to the inquiry have been overcome. In the meantime, Crofts has 
published in the Comptes Rendus the results of an investigation carried out 
according to the method proposed by Meyer, and they agree with those of 
Meyer which had not yet been made known. This has led Victor Meyer 
to publish his results in a series of short notices in the Rev. deut. Chem. 
Gesellsch. (1880, xiii. 34). They are directed, in the first place, to the com- 
portment of the two other halogens at a red heat, then to the conditions 
under which the dissociation occurs and is persistent, and finally to the cause 
of it. In his paper on chlorine Meyer pointed out that iodine comported 
itself similarly. At 600° the density of iodine vapour corresponded exactly 
with the formula I 2 ; at 800° a considerable diminution of the density was 
observed, and from 1027° to 1567°, through a range of temperature extending 
over 500°, it was fixed and unalterable corresponding to § I 2 . Iodine differs 
from chlorine in that the decrease of density, amounting to is reached at a 
lower temperature: chlorine only at 1200° is completely reduced to § 
Cl 2 , while iodine shows the complete change at about 1000°. Bromine 
showed the same change at 1570°, when the density fell to 3*78 and 
3*64 ; the theoretical density of Br 2 is 5*52, and of §Br 2 3*64. It there- 
fore follows the same law, its density diminishing to the extent of |rd 
its normal value at a yellowish red heat. The dissociation of chlorine 
to molecules of the size § Cl 2 , which when nascent chlorine from platinum 
protochloride is employed occurs at a yellow red heat, does not take 
place at the same temperature when ready-prepared chlorine is employed 
in its place; iodine, on the other hand, undergoes the change in either case. 
The capability of CL, and § Cl 2 of existing at the same temperature, which 
the foregoing statement implies, is analogous to that of oxygen and ozone, 
which, as regards their dissociation, comport themselves in exactly the same 
manner. It is not a little curious, that while Deville and Troost noticed no 
change in the density of iodine vapour at 1000°, both Meyer and Crofts have 
observed the diminution of |rd its density at that temperature : the former 
places the iodine in a cold vessel and slowly heats it ; the latter places it at 
once in the highly heated apparatus. 
GEOLOGY AND PALAEONTOLOGY. 
Pre- Cambrian Rocks in the Scottish Highlands . — According to Dr. Hicks, 
in a paper communicated to the Geological Society, the North Western and 
Central Highlands of Scotland include the following districts in which the 
rocks are wholly or in part pre-Cambrian : — 
(1) Glen Finnan , Loch Shiel to Caledonian Canal . — In the former district 
