RECENT ADVANCES IN SCIENCE 19 



ing its rate of ascent had any effect upon the velocity. This 

 cotton was unwound by the balloon as it rose, and the interval 

 between the times when two small pieces of white paper, 

 attached at a measured distance apart, left the ground was 

 timed by means of a stop-watch. For balloons whose weight 

 varied between 11 gm. and 21 gm., the mean value of q with 

 cotton was 8 1*3, and without cotton 83*2, the difference being 

 greatest for the smaller balloons. Lastly the effect of hanging 

 a small lantern 2 metres below the balloon was studied. The 

 weight of the lantern was of course added to the weight of 

 the balloon when calculating q. It was found, from a number 

 of experiments made on different days, that the mean for q 

 without a lantern was 83*5 and with a lantern 83'2, so that the 

 lantern had no appreciable effect. The explanation offered 

 for this is that the increased air-resistance offered by the 

 lantern is practically compensated for by the steadying effect 

 of adding this small load, to which allusion has already been 

 made. In conclusion, mention should be made of a result 

 obtained from many observations, namely, that for balloons 

 weighing between 11 gm. and 16 gm. q = 80, and for those 

 between 20 and 35 gm. q = 84, on an average. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A.. D.Sc, 

 University, Liverpool. 



Resonance Potentials and lonisation Potentials of Hydrogen. — 

 The behaviour of hydrogen molecules and atoms when bom- 

 barded by electrons of various velocities, has been investigated 

 recently by several independent observers. The importance of 

 such investigations hes in the fact that they afford a means of 

 obtaining information regarding the structure of the hydrogen 

 molecule and atom. The conclusions arrived at may then be 

 employed to test the various theories of molecular and atomic 

 structure which have been proposed. It is somewhat remark- 

 able that, in spite of the care and precautions with which these 

 independent investigations have been made, the results are by 

 no means wholly concordant. Nevertheless sufficient progress 

 has been made in this field to warrant a brief summary of the 

 conclusions. Incidentally it may be recalled that a resonance 

 potential denotes the energy of an electron which is capable of 

 bringing about a change in the molecule of the gas (hydrogen) 

 in which, however, no electron is set free, i.e. a change such as 

 H2->2H, the dissociation of the molecule into electrically neutral 

 atoms. On the other hand, an ionisation potential represents 

 the energy which must be imparted to an electron in order 

 that, by its collision with an atom or molecule, it may cause 

 an electron to be eliminated, i.e. the processes : H2 -^ H2 "^ + 0, 

 or H->H+-l-0. 



