SCIENCE GOSSIP. 



185 



COXTHIBUTED BY W. H. CADMAN. 



The Triple Point. — The only point at which 

 the three conditions, solid, liquid, and vapour, can 

 exist together is called the " triple point." This 

 point becomes quite intelligible when we consider 

 the curves with reference to these three conditions 

 indicated on the diagram below. Looking at the 

 boundary lines between solid and liquid, liquid 

 and vapour, and vapour and solid, it will be seen 

 that they all start from one point. This repre- 

 sents the melting-point under maximum vapour 

 pressure, the cu.stomary melting-point under atmo- 

 spheric pressure being slightly higher, though the 

 difference is practically unnoticeable. 



In the diagram pi-essure p is taken as ordinate, and tempera- 

 ture T as abscissa, t is tlie triple point. 



The graphic figure of these states is very in- 

 structive. It consists of (1) areas, (2) lines that 

 bound these areas, and (3) a point in which the 

 lines meet. The areas indicate the conditions 

 under which the substance is present only in one 

 state, whether solid, liquid, or vaporous ; the lines 

 indicate those in which two states are possible ; 

 finally, under the conditions indicated by the point 

 of intersection all three states, solid, liquid, and 

 vapour, can exist side by side. The boundary 

 between liquid and vapour vanishes at the critical 

 point. If we ijrolong the rising curve between 

 solid and liquid it is possible that at some tempera- 

 ture and pressure solid and liquid also lose their 

 sharp distinction, and we should then have an 

 amorphous half-liquid, half-solid state. Spring 

 appears to have passed this point. He prepared 

 '■ Lehmann's flowing crystals " by exposing powder 

 of solid metals to a pressure of more than 1,000 

 atmospheres. This gave, by its homogeneity and 

 crystalline structure, the impression of having 

 been melted. When these facts and their graphical 

 representation are treated from the thermo- 

 dynamical relation the well-known reversible cycle 

 process applied to evaporation is available, and we 

 get a thermo-dynamic expression regulating the 

 three curves. 



An Apparent Paradox explained by 

 Electrical Oscillations. — The result of an 

 experiment by M. Pellat appears at first sight 

 paradoxical, but is easily explained by electrical 

 oscillations. Two condensers of very unequal 



capacity (a battery of six large jars and a small 

 Leyden jar, for example) have their armatures con- 

 nected through an invertor which enables the 

 communication to be alternated. Two discharging 

 tongs are placed near the small condenser, and 

 they allow a spark to pass as soon as the difference 

 of potential between the armatures is sufficient. 

 If the two condensers are then charged with only 

 half the charge necessary to produce a spark, or 

 even a little less, and if the communications of the 

 armatures be inverted by working the invertor, 

 then the spark passes between the discharging 

 tongs. Now, it might be supposed that, if the 

 spark did not pass after the inversion and the 

 state of equilibrium had been attained, the 

 difference of potential between the armatures 

 would be diminished, since the inversion put the 

 positive armature of one of the condensers into 

 connection with the negative armature of the other, 

 and vice versa. As a matter of fact, the difference 

 of potential of the armatures of tlie small condenser 

 has more than doubled at a certain instant by 

 following electrical oscillations. 



Thunder and Lightning. — Lightning is caused 

 by the equalisation of potential in the clouds, 

 where the electrical spark produces the lightning, 

 and the accompanying sound appears as thunder. 

 There are three forms of lightning — fork lightning, 

 sheet lightning, and ball lightning. Slieet light- 

 ning may be regarded as brush-like discharges 

 from cloud to cloud. Fork lightning may be con- 

 sidered as a spark with ramifications. The quantity 

 of electricity contained in a cloud depends upon 

 its capacity and potential. The difference of 

 potential between tw'o clouds, or between the 

 cloud and the earth, may become so great that the 

 intervening medium of air gives way under the 

 strain, and a flash of lightning is the result. The 

 reader will see how closely this resembles the 

 oscillatory discharge of a Leydon jar having a thin 

 glass dielectric between the two coatings. When 

 the jar is charged sufficiently the difference of 

 potential between the outer and inner coatings 

 becomes so great that the thin glass medium is 

 unable to bear the strain, the glass is pierced, and 

 a spark passes between the two coatings. Lightning 

 invariably traverses the path of least resistance. 

 Hence the great value of metallic lightning con- 

 ductors for the protection of buildings. The lower 

 end of a lightning conductor should not merely 

 pass into the ground. It should, if possible, be 

 connected with water-pipes, or else pass into a 

 specially prepared bed at some distance from the 

 building. Ball lightning, a phenomenon rarely 

 met with, consists of balls of fire visible for about 

 ten seconds and then bursting with a loud ex- 

 plosion. Lightning without thunder may be a 

 quiet flowing-out of electricity from the clouds, or 

 possibly a reflection of a far-distant thunderstorm. 

 The time between the flash of lightning and the 

 accompanying thunder enables us to approxi- 

 mately determine the distance of the thunder- 

 storm. Sound travels about 1,100 feet per second. 

 Light from that distance reaches us in such a 

 small time that we may neglect it. The thunder- 

 storm will therefore be at a distance of 1,100 feet 

 X the number of seconds between the time of 

 seeing the flash and hearing the report. With 

 some practice and a clearly indicating seconds- 

 hand on a watch it should he found easy to esti- 

 mate the distance between the cloud originating 

 the flash and the observer. 



