SCIENCE-GOSSIP. 



255 



CONTRIBUTED BY W. H. CADMAN. 



Velocity of Sound in Hot Air. — A recent 

 experiment has been made by E. H. Stevens. He 

 used a porcelain resonance tube closed at one end, 

 which was heated by means of a coal-stove. The 

 actual velocity of sound in the hot air at 950° and 

 1,000° was found to be slightly less than the 

 theoretical value. In each case the value 1-34 was 

 obtained for the ratio of the two specific heats 

 from the observed velocity of sound in the hot air, 



using Laplace's formula V = . /Z_. Determina- 

 tions made in Arctic expeditions at very low 

 temperatures below the freezing-point give values 

 for the velocity of sound in cold air also slightly 

 less than the calculated value. 



Electric Convection.— In the October number 

 of Science-Gossip a short account was given of 

 the experiments by means of which M. V. Cremieu 

 concluded that a moving electrified body produces 

 no magnetic effect, though, in opposition to the 

 classical experiment of Professor Rowland, Harold 

 Pender has just published an account of his 

 elaborate research on this subject. His experi- 

 ments show beyond any doubt that electric con- 

 vection does produce magnetic action. For a 

 complete description of Pender's experiments we 

 refer our readers to the " Physical Review " for 

 October. 



Period of a Rod Vibrating in a Liquid. — 



The ' : Physical Review " for September contains an 

 account of a series of experiments on the period of 

 a rod vibrating in a liquid by Mary I. Northway 

 and A. Stanley Mackenzie. It has been known 

 since 1786 that the period of a pendulum is greater 

 when vibrating in a fluid than when vibrating in a 

 vacuum. This is due to loss in weight through 

 buoyancy and also to the pendulum being loaded 

 by, or dragging with it, a certain amount of the 

 fluid. In 1848 Stokes treated the problem of a 

 body oscillating in an infinite liquid medium, and 

 considered mathematically the effect of the vis- 

 cosity as well as of the density of the fluid. The 

 experiments about to be described approximate 

 very closely in their main results to this important 

 piece of work. The relation between the pitch of 

 bells and tuning-forks in air and that in various 

 liquids has received the attention of several experi- 

 menters, and various suggestions have been made 

 to explain the lowering of pitch from simple 

 mechanical principles. The rod experimented on 

 was held in a clamp screwed to the inside of a 

 wooden box, which held when required the liquid 

 under consideration. An electro-magnet was used 

 to maintain the rod in vibration. The experiments 

 were made in water, cotton-seed oil, and a satu- 

 rated solution of sodium nitrate with brass and 

 steel rods. The results show that the interval of 

 lowering for a rod of given cross-section is in- 



dependent of the length, and is also approximately 

 the same for brass and steel. It is further evident 

 that the main factor in the lowering of pitch is 

 the density of the medium, and that the effect of 

 viscosity is relatively small. 



The Pressure of Light. — Professor Peter 

 Lebedew of Moscow University gives in the 

 "Annalen der Physik " for November last an 

 account of a research by means of which he has 

 succeeded in demonstrating experimentally the 

 pressure of light. Maxwell pointed out that " the 

 concentrated rays of the electric lamp falling on 

 a thin metallic disc, delicately suspended in a 

 vacuum, might, perhaps, produce an observable 

 mechanical effect. It was thought that Sir 

 William Crookes obtained this effect in his radio- 

 meter, but the magnitude proved many thousand 

 times too great. Professor Lebedew used a large 

 bulb with high exhaustion and excluded rays 

 capable of heating the tube walls. In this way he 

 eliminated the radiometer action. The radio- 

 meter vanes were of very thin aluminium foil 

 suspended by a glass fibre, and the electric arc 

 was the source of light. The following are the 

 principal results : — (1) The incident beam of light 

 exerts a pressure both upon an absorbing and a 

 reflecting body ; (2) this pressure of light is 

 directly proportional to the amount of incident 

 energy and is independent of the colour of the 

 light ; (3) this pressure of light corresponds with 

 the ponderomotor, forces of radiation calculated 

 by Maxwell and Bartoli. Hence the existence of 

 the Maxwell-Bartoli pressure of light is experi- 

 mentally proved. 



The Rainbow. — When a ray of sunlight falls on 

 a raindrop it is refracted, then part of the light is 

 reflected from the internal surface and again 

 refracted on leaving the drop. In the figure the 

 path of such a ray has been drawn. The white 

 sunlight is not only refracted when it enters 



Red 



Fig. 1. Showing Formation 

 of Primary Bow. 



Violet 



Violet Red 



Fig. 2. Showing Formation 

 of Secondary Bow. 



and leaves the drop, but dispersion also takes 

 place as shown by the dotted lines. The eye sees 

 bright circles of light for each kind of light, and 

 since sunlight is made up of different kinds of 

 homogeneous light we get a series of circular arcs, 

 showing the spectrum colours, the red being out- 

 side, and the other colours following in the order 

 of descending wave-length. The whole constitutes 

 a primary rainbow. A secondary bow is sometimes 

 seen outside the first. This is formed by the light 

 being twice reflected inside the raindrops in the 

 manner shown in the second figure. The less 

 refrangible rays are on the inner side. Rainbows 

 due to still more internal reflections are too feeble 

 to be observed. It is possible to get a white rain- 

 bow if the sun is clouded or if there is a mixture of 

 raindrops of very different sizes. 



