unit of weight equal to one billionth of one billionth of a gram, and then on this scale the hydrogen molecule weighs 10 such units. We may obtain in another way an illustration of the mass size and number of the molecules of any gas in the following manner:
First as to size. We can in a good Whitworth measuring instrument detect a variation in length of a metal bar equal to one millionth of an inch. This short length would be occupied by 25 molecules placed in a row close together. We can in a good microscope see a small object whose diameter is one hundred-thousandth of an inch. In a small box of this size we could pack 16 million molecules close together. The smallest weight which can be weighed on a very good chemical balance is one hundredth of a milligram. The united weight of one million million million molecules of hydrogen would therefore just be detectable on such a balance.
Until a few years ago our knowledge of the divisibility of matter may be said to have ended with the chemical unit, the atom. But of late years information has been steadily accumulating which has made us acquainted with matter in a finer state of subdivision. For a long time a controversy was carried on, whether the radiation in a high vacuum tube which proceeds from the kathode was a material substance or a wave motion of some kind. But no fact yet found is inconsistent with the notion which originated with Sir William Crookes that the transfer which takes place is that of something which has the inertia quality of matter, and his term 'radiant matter' is a peculiarly suitable phrase to describe the phenomena. The great advance which has since been made, by Professor Thomson and others, is that of measuring accurately the amount of bending which a stream of this radiant matter experiences under a known magnetic force and from this deducing the ratio between the mass of the radiant particle and the electric charge carried by it. This measurement shows that if the radiant matter consists of corpuscles or particles, each of them carries a charge of one electron, but has a mass of about one thousandth of a hydrogen atom.
The evidence therefore exists that Crookes' 'radiant matter' also called the 'kathode rays' and Thomson's 'corpuscles' are one and the same thing and that these corpuscles may be described as fragments broken off from chemical atoms and possessing only a small fraction of their mass. These particles are shot off from the negative terminal or kathode of the vacuum tube with a velocity which is from one fifth to one third the velocity of light.
Moreover it has been shown that when the kathode rays pass through a thin metal window in a vacuum tube and get into the space