MOLECULAR PRESSURE, AND THE TRAJECTORY OF MOLECULES. 
1G3 
be ignited, show that the ignition takes place at a less high exhaustion in hydrogen 
gas than it does in air. 
I have great pleasure in expressing my continued obligation to the great skill in glass- 
blowing and manipulation possessed by my friend and assistant, Mr. C. H. Gimingham, 
whose dexterity in executing complicated forms of apparatus has rendered easy a 
research which otherwise would have been full of difficulties. 
Fig. 22. 
O 
584. I hope I may be allowed to record some theoretical speculations which have 
gradually formed in my mind during the progress of these experiments. I put them 
forward only as working hypotheses, useful, perhaps necessary, in the first dawn 
of new knowledge, but only to be retained as long as they are of assistance ; for 
experimental research is necessarily and slowly progressive, and one’s early provisional 
hypotheses have to be modified, adjusted, perhaps altogether abandoned in deference 
to later observations. 
AN ULTRA-GASEOUS STATE OF MATTER, 
585. The modern idea of the gaseous state of matter is based upon the supposition 
that a given space of the capacity of, say, a cubic centimetre, contains millions of 
millions of molecules in rapid motion in all directions, each having millions of 
encounters in a second. In such a case the length of the mean free path of the 
molecules is excessively small as compared with the dimensions of the vessel, and 
properties are observed which constitute the ordinary gaseous state of matter, and 
which depend upon constant collisions. But by great rarefaction the free path may 
be made so long that the hits in a given time are negligible in comparison to the misses, 
y 2 
