In a communication to M. Dumas, received two days after the above sitting, M. Pictet described his experiments more fully, prefacing the account by the following very interesting remarks:--"The end to which I have been tending for the last 3 years has been to seek to demonstrate experimentally that molecular cohesion is a general property of bodies without exception.
"If the permanent gases cannot be liquefied, it must be concluded that their constituent particles do not attract each other, and are therefore independent of this law.
"To succeed experimentally in bringing the molecules of a gas into the closest possible proximity, and thus to obtain its liquefaction, certain indispensable conditions are necessary, which I thus sum up:
"1. To have a gas that must be perfectly pure and without a trace of foreign gas.
"2. To have at one's disposal very powerful means of compression.
"3. To obtain an intense degree of cold, and the abstraction of heat at these low temperatures.
"4. To have a large surface of condensation maintained at these low temperatures.
"5. To have the power of utilising the expansion of the gas under considerable pressure to the atmospheric pressure, which expansion added to the preceding means compels liquefaction.
"With these five conditions fulfilled, we may formulate the following problem.
"When a gas is compressed at 500 or 600 atmospheres, and kept at a temperature of -100° or 140°, and then let expand to the pressure of the atmosphere, one of two things must occur. Either the gas, obeying the action of cohesion, liquefies and yields its heat of condensation to the portion of the gas, which expands and is lost in the gaseous form; or under the hypothesis that cohesion is not a natural law, the gas passes beyond absolute zero, that is to say, it becomes inert, a dust without consistence. The work of expansion would be impossible, and the loss of heat absolute."
Spite of M. Cailletet's supposition that nitrogen would prove a very incoercible gas, his experiments showed the contrary, since he found that it easily condensed under a pressure of about 200 atmospheres and at a temperature of 13° C., the conditions as to its sudden expansion being observed.