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the thermometers in this shaft (which was dry) has vitiated the observations, the instruments employed being maximum thermometers. Two of the slow non-registering thermometers mentioned in last year's Report have been sent to M. Delesse, to be used for verification.

The slow-action thermometers are constructed on the following plan :The bulb is cylindrical and very strong, and is surrounded by stearine or tallow, which fills up the space between it and a strong glass shield in which the thermometer is inclosed. The shield is not hermetically sealed (not being intended for protection against pressure), but is stopped at the bottom with a cork, so that the thermometer can be taken out and put in again if desired. Stearine and tallow were selected after trials of several substances, including paraffin-wax, bees'-wax, glue, plaster of Paris, pounded glass, and cotton-wool. The thermometers are inclosed in copper cases lined with india-rubber. When placed, without these cases, in water differing 10° from their own temperature, they take nearly half a minute to alter by one tenth of a degree.

In concluding this Report, your Committee desire to express their regret at the losses which they have sustained by the deaths of Prof. Phillips, Sir Charles Lyell, and Col. Strange, of whose valuable services they have been deprived within the last three years.

Nitrous Oxide in the Gaseous and Liquid States.
By W. J. JANSSEN.

[A communication ordered by the General Committee to be printed in extenso.] THE experiments of Faraday on the liquefaction of gases have already proved that gases at the ordinary conditions of pressure and temperature are vapours at a remote stage from their points of condensation. If several gases submitted to great pressure and the cold of the carbonic acid and ether bath did not exhibit any appearance of liquefaction, the cause is probably that Faraday did not obtain a temperature low enough to produce liquefaction. Hence we may conclude that the gaseous and liquid states of matter depend only on the temperature and pressure to which it is exposed. The interesting experiments of Dr. Andrews with carbonic acid (Philosophical Transactions for 1869) not only verified this conclusion, but gave the important result that gases and liquids are distant stages of the same condition of matter, which may pass into one another without breach of continuity. The temperature at which matter, without sudden change of volume or abrupt absorption of heat, passes from the ordinary liquid to the ordinary gaseous. state is called by Dr. Andrews the critical point; above that temperature a gas never can be liquefied by pressure, it behaves like a permanent gas; below that temperature it will be liquid or gas, or more exactly liquid or vapour, according to the pressure to which it is exposed. For the details I refer to the above-mentioned paper.

I have made the same kind of experiments with nitrous oxide, a gas whose physical properties agree much with those of carbonic acid. The apparatus was similar to that used by Dr. Andrews, to whom I am much indebted for the great kindness with which he has afforded me every instruction, and for his invaluable advice about the use of his apparatus during my stay at Belfast and afterwards.

As my experiments with nitrous oxide presented anomalies which did not occur with carbonic acid, I first made some experiments with the latter gas, in order to try whether they were to be ascribed to observational errors or to the nitrous oxide I used. The results are given in the following Tables, where is the fraction representing the ratio of the volume of the air after and before compression to one another at the temperature t, e the corresponding fraction for the carbonic acid at the temperature t', and 7 the number of volumes which 17,000 volumes of carbonic acid, measured at 0° and 760 millims., would occupy at the temperature and pressure of the observation. The number 17,000 has been taken as unit to compare these Tables with those of Andrews.

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These results agree closely with the experiments of Dr. Andrews at the corresponding temperatures, the differences being only 0.2 of an atmosphere. At 210-47 the gas passed into the liquid state at a pressure of 59.8 atmospheres, whilst its volume had diminished from 17,000 to 162; with Dr. Andrews this pressure amounted to 60.05 atmospheres, and the corresponding volume of the carbonic acid to 160. As the quantity of air in my case was about of the entire volume of the gas, the increase of pressure to liquefy the whole after liquefaction had begun, amounted to about 2-4 atmospheres, viz. from 59.81 to 62-18. The critical temperature I found to be 30°-87. It will be observed that the pressures are those indicated by the apparent contraction of the air in the air-tube.

In the following Tables and e have the same meaning as before, but applied to nitrous oxide; 1, however, represents the number of volumes which 1000 volumes of nitrous oxide, measured at 0° and 760 millims., would occupy at the temperature and pressure of the observation. The experiments were made at the temperatures of 25°15, 32°-2, 36°-4, 38°4, and 43°.8, two series below, and three above, the critical point, which was found to vary between 36°-3 and 36°7. The appearances were the same as with carbonic acid.

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