ure of these objects are simply ingenious modifications of those involved in processes already described. Most of the scientific apparatus in glass is brought from Thuringia. Our own workmen do not seem to have that turn for science which is shown by the Germans. Even the little apparatus which is made in this country is for the most part the work of foreign artisans.
It would be a grave omission to close even so brief a summary of the office served by glass in science without calling attention in a few words to one of the latest and most interesting lines of research which it has made possible. We refer, of course, to the vacuum tubes employed by Mr. Crookes in his well-known investigations into the properties of radiant matter. We have been accustomed
Fig. 9.—Vacuum Tubes.
to talk somewhat glibly about the three states of matter. To this list we are now asked to add a fourth, the radiant. Faraday's hint and the work of Mr. Crookes have well-nigh established the distinction between gas and radiant matter. In a gas under ordinary tension the molecules are in sufficient number to suffer almost constant collision with one another; but if the tension be low enough, say the one five-millionth of an atmosphere, the collisions become infrequent, and the molecules travel in almost uninterrupted straight lines until they come into con-
