Dec. 14, 1882] 
NATURE 
147 
importance. The space devoted to the other ‘Cata- 
racts” of the world is small, though most of the impor- 
tant ones are mentioned. The illustrations are good, and 
on the whole the book is interesting. 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents. Neither can he undertake to return, 
or to correspond with the writers of, rejected manuscripts. 
No notice is taken of anonymous communications, 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it is impossible otherwise to ensure the appearance even 
of communications containing interesting and novel facts.] 
Priestley and Lavoisier 
Ir Mr. Rodwell had anything new to tell us about Lavoisier, 
there would have been a sufficient motive for his writing ; but I 
do not see what useful purpose is gained by telling us what was 
already known, namely, that a century ago Lavoisier rendered 
many important services to science ; or, what was not so well 
known, namely, that chemistry is a French science ; or, that 
Lavoisier was ‘‘the most generous of men,” ‘‘incapable of any 
meanness.” The real question Mr. Rodwell himself asks :— 
“Upon what authority does Dr. Thomson assert that Dr. 
Priestley informs us that he prepared the gas in M. Lavoisier’s 
house in Paris, and showed him the method of procuring it in 
the year 1774?” 
Mr. Rodwell quotes from Thomson’s notice of Priestley ; had 
he turned to that of Lavoisier (p. 105, vol. ii. 1831, not 1830), 
he would have found an answer :—‘‘ Dr Priestley discovered 
oxygen in August, 1774, and he informs us in his life [this ought 
to be ‘Life,’ ze, autobiography] that in the autumn of that 
year he went to Paris and exhibited to Lavoisier, in his own 
laboratory, the mode of obtaining oxygen gas by heating red 
oxide of mercury in a gun-barrel, and the properties by which 
this gas is distinguished; indeed, the very properties which 
Lavoisier himself enumerates in his paper. [A/em. Acad. 1775, 
pub, 1778.] There can therefore be no doubt that Lavoisier 
was acquainted with oxygen gas in 1774, and that he owed his 
knowledge of it to Dr. Priestley.” 
Dr. Black complained of the publication of Lavoisier’s papers 
without any allusion whatever to what he himself had previously 
done on the same subject. Cavendish complained of something 
more than a similar neglect. The facts, as stated in Dr. George 
Wilson’s ‘‘ Life of Cavendish,”’ are briefly these :—Blagden went 
to Paris in June, 1783, and informed Lavoi-ier of the discovery 
of the composition of water. Lavoisier was incredulous, ex- 
pressing his opinion that the union of the two gases (O and H) 
would produce, not water, but an acid. Nevertheless he re- 
peated Cavendish’s experiment on a large scale; and in his 
account of it to the Academy on June 25, stated that the con- 
clusion as to the compound nature of water was drawn by 
Laplace and himself. The charge brought against Lavoisier by 
Cavendish, Blagden, and Watt, was summed up by Watt to this 
effect, that after Lavoisier had had the theory of the composi- 
tion of water explained to him, ‘he invented it himself.” 
Mr. Rodwell ‘‘ utterly denies” that the acceptance of Lavoi- 
sier’s doctrine was mainly due to Cavendish’s discovery. A 
strong objection to the oxygen theory was advanced by Ber- 
thollet and others, founded on the observation that in the action 
of dilute acids on metals inflammable air is produced. [The 
inflammable air of Cavendish, in 1766, was referred not to 
water, but to the metals]. Whence came this element? ‘The 
discovery of the composition of water answered the objection, 
and converted it, as Dr. Whewell remarks, into an argument in 
favour of the theory. 
My statement that ‘‘the compound is always equal to the sum 
of its elements” was already known, was elicited by a remark of 
Lavoisier’s, quoted by Mr. Rodwell :—‘‘I am obliged in this 
reasoning to suppose that the weight of the bodies employed was 
the same after the observation as before.” My statement is new 
to Mr. Rodwell, and he calls for references. Many of the old 
writers on the idea of substance acknowledged the proposition, 
and its best application was Wenzel’s doctrine of definite and 
reciprocal proportions, although its full significance did not be- 
come apparent until the aeriform elements were also taken into 
account. 
But to return to Priestley, I am bound to admit that 1744 
is a mistake into which I was misled by Whewell (‘‘ His. Ind. 
Sci., 1857, iii. 110), who gives that date. 
Priestley was presented with the Royal Society’s Copley 
medal, as an honourable testimony to his numerous scientific 
discoveries, which, considering the crude state of chemistry in 
his time, must be regarded as admirable. He was afterwards 
driven from the Royal Society and from his country, his house 
was pillaged, and his library, manuscripts, and apparatus de- 
stroyed, and all this persecution was on account of certain 
opinions which happily are now widely spread. The statue at 
Birmingham is a less impressive tribute to his memory than the 
maintenance of respect for his fame ; andit is with no unfriendly 
feeling towards Mr. Rodwell that I express an opinion that this 
old quarrel between Lavoisier, Priestley, and Cavendish had 
better be left to repose in the history of science, where it has 
been discussed with sufficient fulness and fairness by such 
writers as Thomson, Brande, Whewell, and George Wilson. 
Highgate, N., December 4 C, TOMLINSON 
The Forth Bridge 
IN some remarks made in NATuRE, vol. xxvii. p. IOI, by 
Mr. Charles Shaler Smith, the following passage occurs :— 
‘*The tests of the last few years show conclusively, that iron 
exposed to compression within its buckling limit is compacted 
in texture and strengthened by such use while, if subjected to 
continuous tension beyond two-thirds of its elastic limit, it ‘is 
attenuated and weakened.” As I think that the words above 
quoted may perhaps to a certain extent mislead those who have 
not themselves made experiments on the elasticity of iron and 
steel, and on the alteration of density which can be produced by 
compression or extension, I would observe :— 
1. That the increase of density which can be produced by 
compressing within the buckling-limit such rods as may be em- 
ployed in the construction of bridges, would certainly not 
account for the strengthening of iron exposed to continuous 
compression. J have examined carefully the alteration of density 
which can be effected in iron and steel wires by Jongitudinal 
extension, &c., and even in cases where the wire was strained to 
breakage, and the permanent extension exceeded 20 per cent., 
there was no diminution of density equal to 1 per cent. Of 
course the words ‘‘compacted texture” may not mean that the 
density is increased, but the idea seems to be not uncommon 
among engineers, that increase of strength mecessarz/y implies 
increase of density. Though I cannot at this moment lay hands 
upon it, I remember reading an account of some theories ad- 
vanced respecting the hardening of steel, from which it was 
evident that the author of these theories assumed that the 
hardening is attended with zucrease of density, whereas the 
density of steel can be more diminished by this process than by 
any mechanical means with which I am acquainted. 
2. It is quite true that iron, if subjected to continuous tension 
beyond two-thirds of its elastic limit, is attenuated, but whether 
the attenuation is attended with weakening or not depends 
largely upon the manner in which the tension is applied. If the 
latter is increased by small amounts at a time, and each amount 
allowed to act for a few hours before any increase of stress is 
made, not only is there comparatively ‘small permanent exten- 
sion, but there may be an actual increase of strength as far as 
resistance to extension is concerned. ‘The fact is that whether 
we subject iron and steel to domg-continued compression or exten- 
sion, we increase the resistance 'to compression and extension 
respectively, mainly for the same reason, namely, that we give 
time for the molecules of the metal to take up such positions as 
will enable them to offer the maximum resistance. Thus I have 
proved that the value of ‘‘ Young’s Modulus” is considerably 
increased in the case of an iron wire which has suffered per- 
manent extension, by allowing the wire to rest for some hours 
either loaded or unloaded ; this increase of elasticity is not 
attended with any appreciable increase of density. 
As I feel that too much precaution cannot be taken in a 
question of this kind, where life is at stake, I would venture to 
make the following suggestion:—That bars or rods of steel 
and iron which run the s/gh/est risk of having at any time to 
undergo a considerable extending or compressing stress should 
before use be subjected, if possible, to the same kind of stress 
gradually increased in amount with intervals of some hours 
between each increase until a stress equal toat least three-fourths 
of the breaking-stress be reached. Three or four days would 
suffice to bring the metal to its maximum strength, both as 
regards resistance to permanent and to temporary strain. 
