78 
Proceedings of the Royal Society 
Thus it is seen that even the turn of one of Newton’s phrases 
serves, when rightly viewed, to dissipate a widespread delusion : — 
and that while Boyle, though perhaps he can scarcely he said to 
have been “born great,” certainly “achieved greatness;” the 
assumed parent of La Loi de Mariotte (otherwise Mariottesches 
Gesetz ) has as certainly had “ greatness thrust upon ” him. 
3. The Graphic Analysis of the Kinematics of Rigid-Bar 
Mechanisms. By Professor R. Smith. Communicated 
by Professor Tait. 
4. Note on the Necessity for a Condensation-Nucleus. 
By Professor Tait. 
The magnificent researches of Andrews on the Isothermals of 
Carbonic Acid formed, as it were, a nucleus in a supersaturated 
solution, round which an immediate crystallization started, and has 
since been rapidly increasing. 
They gave the clue to the explanation of the paradoxical result of 
Regnault, that hydrogen is less compressible and other gases more 
compressible, under moderate pressures, than Boyle’s Law indicates ; 
and to that of the companion result of Natterer that, at very high 
pressures, all gases are less compressible than that law requires. 
Thus they furnished the materials for an immense step in connec- 
tion with the behaviour of fluids above their critical points. 
But they threw at least an equal amount of light on the liquid- 
vapour question — -i.e., the behaviour of fluids at temperatures under 
their critical points. In Andrew’s experiments there was a com- 
mencement, and a completion, of liquefaction ; each at a common 
definite pressure, but of course at very different volumes, for each 
particular temperature. 
In 1871 Professor J. Thomson communicated to the Royal 
Society a remarkable paper on the abrupt change from vapour to 
liquid, or the opposite, indicated by these experiments. He called 
special attention to the necessity for a “ start,” as it were, in order 
that these changes might be effected. [It is to this point that the 
present note is mainly directed, but I go on with a brief analysis of 
