TRANSACTIONS OF THE SECTIONS. 45 
instead of straight lines, it gers the resulting figures a peculiar twist. To over- 
come this defect a double link motion has been adopted :— 
Pe 
B 
CY er, 
E 
| 
Pe 8 | 
AB and CD are two rods moving freely on their centre, E. At the points A, B, C, 
and D are ball-and-socket joints connected with the tops of four pendulums, 
P,, P., P,, Pj, by means of wire arms. 
e pendulums can be set in motion either in pairs or in threes or all four at once, 
the resulting motions of the tracer E producing very curious figures. 
By this arrangement, if the two opposite pendulums (say P, and P, or P, and P,) 
are set in motion, the other two being fixed, and a strip of paper drawn under the tracer, 
a modification of the curves described by Mr. A. E. Donkin in last year’s Report will 
be produced. 
CHEMISTRY. 
Address by Professor A. Crum Brown, M.D., P.RS.E., F.CS., President of 
the Section. 
One hundred years have elapsed since the discovery of oxygen by Priestley. Per- 
haps we should say rediscovery, for there is no doubt that about one hundred years 
earlier Mayow prepared from nitre nearly pure oxygen, and observed and recorded 
some of its most marked properties. Mayow’s discovery, however, led to nothing, 
while Priestley’s was the most important step in that reconstruction of speculative 
chemistry which was commenced by Black and carried on with surprising energy 
and thoroughness by Lavoisier and his associates. I shall not detain you by enu- 
merating the ways in which this discovery has affected chemistry both practical and 
speculative. The preeminent position to which oxygen was at once elevated, and 
which it so long retained, males this altogether unnecessary. I wish, however, to 
point out one character of the phlogistic controversy which sharply distinguishes it 
from many others. The truth represented by the theory of Phlogiston was not re- 
cognized with sufficient distinctness by the supporters of that theory to give them 
any chance of success in opposition to a band of devoted adherents of a view which 
was clearly understood by all. The phlogistists were completely defeated, and the 
theory ceased to exist. It has been left for chemical antiquaries to pick out, with 
difficulty and uncertainty, a meaning from the ruins. 
I have mentioned this character because I wish to draw your attention to ancther 
more recent controversy, the result of which was very different. 
