2. The pressure required to liquefy the nitrous oxide and the volume of this gas at the beginning of liquefaction are variable.

The pressure required to liquefy the gas at 25°-15, recorded in Table I., is the mean of the following observations :

The following series of experiments was performed in the course of a

day :

At 32°-2 the greatest difference of pressure amounted to 2 atmospheres, as appears from the next series of experiments.

3. After liquefaction has begun an increase of pressure of 16 atmospheres or more is required to liquefy the whole mass of the nitrous oxide; for at 25°-17 liquefaction began at a pressure of 57.83 atmospheres, whilst the whole was liquid at a pressure of 73.68 atmospheres. At 32°-2 I found the commencement of liquefaction at a pressure of 67.63 atmospheres, and the termination at a pressure of 84.09 atmospheres. For carbonic acid, that was mixed with too of air, the increase of pressure amounted to 1.5 atmosphere. Had the gas been pure no increase of pressure could have occurred. This shows that a greater quantity of a permanent gas must be mixed with the nitrous oxide; the variations of the volume of the gas under a given pressure and temperature result perhaps from its whole mass not being homogeneous, as the diminution of the volume is too fast to allow a perfect diffusion of the two gases.

The gas used for these experiments was prepared from pure nitrate of ammonium. The salt was carefully heated in a tin bath in order to prevent any decomposition of the liberated gas by a too irregular heating when directly exposed to a flame. It was washed by transmission through a strong solution of caustic potash and dried over sulphuric acid. The caustic potash decomposes any solid particles of the salt that might be carried over mechanically and retains the nitric acid, whilst the free ammonia is absorbed by the sulphuric acid. Purified in this manner, the gas was made to pass through the glass tube wherein it was to be compressed. A pressure of about 90 to 100 millims. of mercury was required to maintain a moderate current of gas through the capillary bore: this current was continued for five hours or more in order to ensure the complete removal of the air; the capillary end was then sealed and the other end introduced under mercury. As the experiments with the tube filled in this manner indicated always the presence of a permanent gas, I tried afterwards to remove the air by exhausting the tube with the air-pump and then to fill with the gas; this operation was successively repeated from twenty to thirty times, but with no other result.

As I could not get the gas pure by heating nitrate of ammonium, I tried to get it from liquid nitrous oxide as it is made in iron bottles in London; it was probable that the permanent gas would escape first and the nitrous oxide remain pure. This, however, did not occur, and I got nearly the same result as before.

In order to prevent diffusion as much as possible, all the caoutchouc joints were besmeared with a solution of tar and asphalt, and the current of gas issued under sulphuric acid. The amount per cent. of this permanent gas was determined in the following manner :-The absorption-tube of Bunsen's orptiometer was partly filled under water with nitrous oxide and then left ling three days or longer. The whole of the gas was not absorbed;

there remained a certain quantity, about to of the entire volume, or about 3.5 to 5 per cent.

This permanent gas cannot be nitric oxide nor oxygen; for the current of nitrous oxide being made to pass successively through strong solutions of sulphate of iron and of pyrogallate of potassium, these solutions did not change colour.

The only known permanent gas that could be disengaged is nitrogen. It is a known fact that nitrate of ammonium, in presence of spongy platinum, is decomposed at 160° into nitrogen, nitric acid, and water; the same decomposition of a part of the salt could have been effected by the asperities of the inner surface of the retort. This quantity of nitrogen would exert a considerable influence on the specific gravity of the gas. The theoretical specific gravity of pure nitrous oxide is 1.524; but being mixed with nitrogen to an amount of 3.5 to 5 per cent., it should be found much smaller, 1.504 to 1.496 respectively. This result, however, does not accord with actual experiment. The specific gravity of nitrous oxide, prepared from nitrate of ammonium, was determined according to the method of Bunsen ('Gasom. Methoden,' von R. Bunsen); for that purpose I used a balloon of 200 cubic centims. Four experiments gave the following results:-1-531, 1·525, 1·529, and 1·527: the mean value is 1.528, agreeing very well with the theoretical specific gravity of pure nitrous oxide, but giving a difference of 0.024 to 0.032 from the specific gravity that would have been found if the gas had been mixed with nitrogen. These differences are too large to be accounted for by experimental errors.

An analysis of nitrous oxide was made according to a somewhat modified method of Frankland and Ward. The hydrogen used in these experiments was obtained from the electrolytic decomposition of water, and the oxygen was generated by heating mercuric oxide. To ensure that the mercuric oxide is free from nitrogen, it must be prepared by precipitating corrosive sublimate with caustic potash.

Three analyses of air gave the following satisfactory results :

The following are the results of the analysis of nitrous oxide:

I. Nitrous oxide obtained from the liquid nitrous oxide of an iron bottle.

Hence the volume of the hydrogen 146-23, the volume of the oxygen 57.76, and the contraction after the second explosion 46.69.

The remaining volume (160·19) is a mixture of only nitrogen and oxygen, where the amount of oxygen is 57·76—1 × 46·6942.20; hence the volume of the remaining nitrogen 160-19-42-20-117.99. This volume is by 0.6 larger than the volume of the nitrous oxide used; hence the amount per cent. is 0.52.

The amount of hydrogen that remained after the first explosion is x

46.69-31.12; therefore the amount of hydrogen required to combine with the oxygen of the nitrous oxide is 146-23-31·12=115·11; hence the volume of the oxygen contained in the nitrous oxide is equal to =57.55, differing by 1.96 per cent. from the calculated volume of oxygen, which is 117.39 =58.69.

2

115-11
2

II. Nitrous oxide obtained by heating nitrate of ammonium.

is on page 56.

The only analysis of nitrous oxide I found in Bunsen's Gasom. Methoden' Here Quincke gives the results of an analysis of nitric oxide, to which is added a measured quantity of nitrous oxide in order to effect the explosion.