332 



Dr. Dg La Rue and H. W. Miiller. [Apr. 22,. 



bromine, iodine, sulphur, potassium, sodium, lithium, silver, thallium, 

 aluminium, carbon, phosphorus, barium, calcium, magnesium, and 

 lead, and making a reasonable allowance for the errors of the de- 

 terminations, he calculates the probability that nine of those numbers 

 should lie, as they are found to do, within 0*1 of integers, supposing 

 the value of the true numbers to be determined by chance, and finds it 

 only as 1 to 235*2. The exact figure for the chance will of course 

 depend upon the limit of error taken ; but the above example seems 

 sufficient to show that not only is Prout's law not as yet absolutely 

 overturned, but that a heavy and apparently increasing weight of 

 probability in its favour, or in favour of some modification of it, exists,, 

 and demands consideration. 



IV. " On the Height of the Aurora Borealis." By Warken De 

 La Rue, M.A., D.C.L., F.R.S., and Hugo W. Muller, 

 Ph.D., F.R.S. Received April 22, 1880. 



Our experiments on the electric discharge, which have been already 

 published in the " Phil. Trans." and the " Proceedings of the Royal 

 Society," enable us to state with some degree of probability the height 

 of the Aurora Borealis, when its display is of maximum brilliancy, and 

 also the height at which this phenomenon could not occur on account 

 of the great tenuity of the atmosphere. 



In Part III of our electric researches, " Phil. Trans.," p. 159, vol. 171, 

 we have shown that the least resistance to the discharge in hydrogen 

 is at a pressure of 0*642 millim., 845 M ; after this degree of exhaustion 

 has been reached, a further reduction of pressure rapidly increases the 

 resistance. When the exhaustion has reached 0*002 millim., 3 M, the 

 discharge only just passes with a potential of 11,000 chloride of silver 

 cells (11,330 volts) ; at the highest exhaust we have been able to 

 obtain (and which we believe has not been surpassed), namely, 

 0*000055 millim., 0*066 M, not only did 11,000 cells fail to produce a 

 discharge, but even a 1-inch spark from an induction coil could not 

 do so. 



Although we have not experimentally determined the pressure of 

 least resistance for air, we have ascertained that while the discharge 

 occurs in hydrogen at atmospheric pressure between disks 0*22 inch 

 distant, they require to be approached to 0*13 inch to allow the dis- 

 charge to take place in air. We may therefore assume that the 

 pressure of least resistance for air is 



0*642 x!3 =Q . 379 miUim ? 498 . 6 M 



At a height of 37*67 miles above the sea level, the atmosphere would 



