The results of these calculations tend to show that the boiling point of helium lies below -264° C., at least 20° lower than the value I have found for the boiling point of hydrogen. If the boiling point of a gas be taken as a simple function of its density, helium, which, according to Prof. Ramsay's determination, has a density of 2.133, more than double that of hydrogen, should liquefy at a much higher temperature. Both argon and helium have much lower boiling points than might be expected, judging from their densities. This anomalous condition may be accounted for by the fact that in each case the molecular structure is monatomic, as shown by the values obtained for the ratios of their specific heats. The permanent character of helium might be taken advantage of in its application to the gas thermometer. The helium thermometer could be used to advantage in the determination of the critical temperature and boiling point of hydrogen. To determine whether the hydrogen thermometer is of any value at temperatures below -198° C. I carried out a series of experiments, in which I measured the temperature of liquid oxygen boiling under reduced pressure. I made use of the identical thermometer tube employed by T. Estreicher (Phil. Mag. [5] 40, 54, 1898) as a hydrogen thermometer for the same purpose, and applied the same corrections as were made in his experiments. Temperature. Pressure Helium Thermometer. Hydrogen Thermometer. Mm. Deg. Deg. 741 -182.6 C. -182.6 C. 240 -191.8 C. -191.85 C. 90.4 -198.7 C. -198.75 C. 12 -209.3 C. -209.2 C. 9 -210.57 C. -210.6 C. The results of these experiments prove that the coefficient of expansion of hydrogen does not change between these limits of temperature, and that the hydrogen thermometer is a perfectly trustworthy instrument even when employed to measure the very lowest temperatures. I have already pointed out (Wied. Ann., Bd. xxxi, 869, 1887) that the gas thermometer can be used to measure temperatures which lie even below the critical point of the gas with which the instrument is filled. For instance, the critical temperature of hydrogen, which I have found to be -234.5° C. (Wied. Ann., 56, 133; Phil. Mag. [5] 40, 202, 1898) can be determined by means of a hydrogen thermometer. The helium thermometer could be used at much lower temperatures, and would probably give a more exact value for the boiling point of hydrogen than it is possible to obtain by means of a platinum thermometer. * * * * * SOME NOTES ON SPIDERS. By Rev. SAMUEL BARBER.