PAGE 226 The ether. Its nature considered. Behaves like a gas. 227 Can be pumped out of a receiver. 228 Light and heat do not pass through a tube _in vacuo_. Laboratory experiments examined. 229 Light and darkness in a partial vacuum, though high. 230 Electricity not a carrying agent. 232 Why there are light and dark strata in a high vacuum. 233 The real carrying agent through a high vacuum is the residue of ether left in it. Digression to consider the aurora. 234 How air may be carried to extraordinary heights. Zones of air carried up are made luminous by electricity. 236 Comparison of this method with experiments quoted. 237 Experiment suggested to prove whether light passes freely through a vacuum tube. 238 The ether does not pervade all bodies freely. 239 It must be renounced altogether or acknowledged to be a material body, subject to expansion, condensation, heating or cooling. 239 How light and heat pass through glass. 240 Temperature of the ether variable. Zodiacal light, cause of. THE ETHER A MATERIAL SUBSTANCE, PROVED BY ITS BEHAVIOUR. We have said in a former part of this work, pages 153 and following, that if the ether is capable of performing all the functions that are attributed to it, it must have some consistence or substance of some kind; that it must be matter of some kind in some form, and consequently must have density in some degree however low; and we might, for the same reasons, suppose that it must have some temperature; but as long as we believe that without motion there can be no heat, we cannot conceive it to have any temperature. No doubt we might suppose it to be in a constant state of vibration, and to have the temperature corresponding to that state, whatever that may be; but this, in addition to leaving us just where we were, would only entail upon us the task of supplying temperature as well as density to a body of whose existence no positive proof has hitherto been given, whatever we may believe about it. At the same time, the evident necessity of taking its temperature into consideration, seems to supply another reason for concluding that it is a material substance, over and above those we have cited now and before.