G90 Professor Bewar [June 10, 



On the supposition that this spectrum originates from the formation 

 of some chemical compound, probably formed within certain limits of 

 temperature when vapour of magnesium is in presence of hydrogen, 

 the stability of the body ought to depend largely on the pressure of 

 the gaseous medium. Like Graham's hydrogenium, this body might 

 be formed in hydrogen of high pressure at a temperature at which it 

 would under less pressure be decomposed. In fact, it has been 

 shown by Troost that the hydrides of palladium, sodium, and potas- 

 sium all follow strictly the laws of chemical dissociation enunciated 

 by Deville ; and increased pressure, by rendering the compound more 

 stable, ought, if the secondary elBfect of such pressure in causing 

 a higher temperature in the electric discharge were not overpowering, 

 to conduce to a more continuous and brilliant spectrum of the com- 

 pound. Conversely, if such a more continuous and brilliant spectrum 

 be found to result, in spite of the higher temperature, from increased 

 pressure, it can only be explained by the stability of the substance 

 being increased with the pressure. 



Now, what are the facts ? When the spark of an induction coil, 

 without a Leyden jar, is passed between magnesium electrodes in 

 hydrogen at atmospheric pressure, the flutings in the green are, as 

 before described, always seen, but they are much stronger at the j^oles 

 and do not always extend quite across the field. As the pressure is 

 increased, however, they increase in brilliance and soon extend per- 

 sistently from pole to pole, and go on increasing in intensity, until, at 

 fifteen and twenty atmospheres, they are fully equal in brilliance to 

 the h group, notwithstanding the increased brightness these have 

 acquired by the higher temperature, due to the increased pressure. 

 The second set of flutings, those in the yellowish green, come out as 

 the pressure is increased, and, in fact, at twenty atmospheres only 

 the h group and the flutings are noticeable ; if the yellow magnesium 

 line be visible at all it is quite lost in the brilliance of the yellow 

 flutings. The tail of fine lines of these flutings extend at the high 

 pressure quite up to the green, and those of the green flutings quite 

 up to the blue. On again letting down the pressure the like pheno- 

 mena occur in the reverse order, but the brilliance of the flutings 

 does not diminish so rapidly as it had increased. If, now, when the 

 pressure has again reached that of the atmosphere, a large Leyden jar 

 be interposed in the circuit, on passing the sj^ark the flutings are 

 still seen quite bright, and they continue to be seen with gradually 

 diminishing intensity until the sparks have been continued for a 

 considerable time. It appears that the compound, which had been 

 formed in large quantity by the spark without jar at the higher 

 pressures, is only gradually decomposed, and not re-formed, by the 

 high temperature of the spark with jar. This experiment, which was 

 several times repeated, is conclusive against the supposition that the 

 flutings are merely due to a lower temperature. When the pressure 

 was increased at the same time that the jar was employed, the flutings 



