580 SCIENTIFIC RECORD FOR 1885. 



other simple sul^tances, the number of atoms of which is shown by the 

 density number of each element. He enunciates the law in the follow- 

 ing simple form : " The densities of substances are proportional to the 

 density numbers." Of course it is necessary that the substances com- 

 pared should be under similar conditions ; and then the ratio of the 

 sum of the density numbers to the density is constant. Thus for a 

 series of hydrated crystalline salts of the formula ECI2 (1120)6 this 

 ratio varies from 19-85 to 20-00. Various other applications of the law 

 are given. {Phil Mag., July, August, 1885, V, xx, 15, 191.) 



Lodge, in his lecture on Dust, at the Montreal meeting of the British 

 Association, defined it to be all foreign matter, of whatever kind, sus- 

 pended in the air. Its function in causing the blue color of the sky and 

 the diffusion of sunlight was discussed. If the atmosphere were purely 

 gaseous, holding no minute foreign bodies in suspension, the sun would 

 glare down directly with blinding intensity, and objects not in its direct 

 rays would be in almost complete shadow. The sun would be set in a 

 black firmament, and it would be easy to see the stars at noonday. 

 But so far from this, the sun's rays, on reaching our atmosphere, are par- 

 tially intercepted, diffused, and scattered by myriads of most minute 

 particles, so minute as to be even smaller than the light waves them- 

 selves, which act therefore more powerfully on the smallest of these 

 waves than on the largest. The light thus scattered has a preponder- 

 ance of small waves, owing to the minute size of the scattering parti- 

 cles, and hence it affects our sight organ with the sensation of blue. 

 The function of dust in causing vapor condensation was then considered, 

 and the theory of Aitken illustrated by experiment. Five methods of 

 removing dust from the air were mentioned : (1) Filtration through cot- 

 ton wool, either alone or mixed with glycerin ; (2) settling, especially 

 in hydrogen ; (3) condensing vapor in the air several times ; (4) calcin- 

 ing the air, or keeping a hot body in it for some time; and (5) discharg- 

 ing electricity into it from a ])oint. He points out that the action of a hot 

 body in keeping the dust away from itself is due to molecular bombard- 

 ment; Tait, Dewar, and Reynolds having shown that a Crookes bom- 

 bardment is effective at ordinary pressures, provided the bodies bom- 

 barded are small. Dust particles, being very small, are driven by 

 molecular impact away from hot bodies toward cold ones. Hence we 

 observe that articles in a room warmed by radiation, and therefore 

 warmer than the air of the room, are not likely to become as dusty as 

 those in a room warmed by hot air, where the air is warmer than the 

 articles. The rapid deposition of dust by electrification was shown by 

 discharging the current of a Voss machine from a point into an atmos- 

 phere containing magnesium smoke. Thus, possibly, the air is cleared 

 by thunder storms, and thus fog might possibly be dissipated at sea. 

 {Nature, January, 1885, xxxi,265.) 



Genocchi has published a historical note on the pendulum experiment 

 for determining the earth's rotation. Poleni, iu 1669, in noticing a me- 



