C. GENERAL PHYSICS. 127 



this problem appears to be due to Professor Miiller, of 

 Freiburg. According to him, we obtain a measure of the in- 

 tensity of the glow by dividing the intensity of the galvanic 

 current by the diameter of the wire ; the current intensity 

 being given by Ohm's law, we of course find that the effect 

 will depend upon the number of elements in the galvanic bat- 

 tery, and the electro-motive force of each element; also upon 

 the resistance of the wire and the battery. For the same 

 battery acting on the same length of wire a maximum glow 

 will be produced when the wire has a certain determinable 

 diameter, and the intensity of the glow diminishes when the 

 wire is either thicker or thinner than this. For instance, 

 with six of Ruhmkorff's zinc and carbon elements acting on 

 a platinum wire one decimeter long, the maximum glow is 

 produced when the diameter of the wire is ^ of a millimeter; 

 for a wire two decimeters long the thickness must be y 1 -^ of 

 a millimeter to produce the maximum effect. With a bat- 

 tery of two of Stohrer's elements a platinum wire, two de- 

 cimeters long, can not be raised to a white-hot glow, but 

 may be raised to a red heat when its diameter is 14- millim- 

 eters, or less. Again, in order to make red hot a platinum 

 wire of i millimeter diameter and two meters long, a battery 

 of 28 elements is necessary, while 40 such will not make this 

 wire white hot. JBerichte d. Natwf. Gesell. Freiburg ,VI., 2, 97. 



THE MOLECULAR CONSTITUTION OF GASES AND LIQUIDS. 



That the same substance at the same temperature and 

 pressure can exist in two very different states, viz., as a 

 liquid and as a gas, is a fact of the highest scientific impor- 

 tance, for it is by the careful study of the difference between 

 these two states and the phenomena which occur at the sur- 

 face which separates the liquid from its vapor that we may 

 expect to obtain a dynamical theory of liquids. A dynam- 

 ical theory of perfect gases is already in existence ; that is 

 to say, we can explain many properties of gaseous bodies by 

 supposing their molecules to be in rapid motion, and that 

 they act on one another only when they come very near or 

 strike each other ; but we can not extend this dynamic theory 

 from the rarer to the denser condition obtained by subject- 

 ing the gas to great pressure without at the same time ob- 

 taining some definite conception of the nature of the action 



