ynne 20, 1872] 



NATURE 



H3 



wave-length and the emitted wave-length, so is the velocity of 

 light to the required velocity, to or froai the ohservsr. 



A. CowPER Ranyard 



PROF. CANNIZZARO'S FARADAY LECTURE 



THIS lecture was delivered on May 30, by Prof. 

 Canr>izznro. The lectureship was founded by the 

 Chemical Society in honour of the illustrious Faraday, 

 to be held by some eminent foreign savant, who, during 

 the term of his tenure is to deliver a discourse before the 

 Society. Dr. Frankland, in introducing the lecturer, said 

 that in 1S69, M. Dumas had honoured them v.-ith his pre- 

 sence there, and on that night they were to listen to Prof. 

 Cannizzaro, of P.ilermo. After alluding to the numerous 

 investigations which the Professor had made in organic 

 chemistry, and amongst others the discovery of benzylic 

 alcohol, the first normal aromatic alcohol that had ever been 

 prepared, and to the important theoretical views which he 

 had originated, the President, in the name of the Society, 

 presented to him the Faraday Medal, struck in honour of 

 his visit. 



Prof. Cannizzaro said that when he received the flatter- 

 ing invitation to deliver the Faraday Lecture, he was 

 placed in very unfavourable circumstances to respond to 

 it, as he had no definite results to lay before the Society, 

 and was, moreover, on the point of suspending his labours 

 and abandoning his old laboratory in order to remove to 

 Rom?, and establish a new one there. In this difficulty 

 a subject for a discourse fortunately presented itself, one 

 which the celebrated French chemist, Dumas, had pro- 

 mised to treat of in 1S47, namely, the form which the 

 theory of chemistry should take at the present time. 

 Although this could not be fully discussed in so short a 

 space of time, it would at least have the advantage of 

 directing the attention of chemists to a question of great 

 importance in the transition stage which our science is at 

 present going through. 



In recalling the promise which M. Dumas had made to 

 the Academy of Sciences of Paris in 1847, to examine 

 the form which theoretical instruction in chemistry should 

 take in the present state of the science, the lecturer pro- 

 posed to consider in his discourse the limits within which 

 the exposition of general theories should be included in 

 teaching chemistry, and the form that it was desirable 

 that they should assume. Whiht giving a broad sketch of 

 the progress of modern chemistry, he showed that the 

 atomic theory had become more and more intimately in- 

 terlaced with the fabric of chemistry, so that it is no 

 longer possible to separate them without rending the 

 tissue, as it were, of the science ; and that up to the pre- 

 sent time we have been unable to enunciate even the 

 empirical laws of chemical proportion, independently of 

 that theory ; for those who employ the term equivalent in 

 the sense that WoUaston did, commit an anachronism. 

 Consequently, in the exposition of the value and use of 

 symbols, formute, and chemical equations, not only are 

 we unable to do without the atomic and molecular theory, 

 but it is inconvenient to follow the long and fatiguing 

 path of induction which leads up to it. By one of those 

 bold flights of the human mind we can at once reach the 

 height whence we discern at a glance the relations between 

 facts. 



He then went on to show that the solid basis, the 

 corner-stone of the modern molecular and atomic theory, 

 the crown of the edifice of which Dalton laid the founda- 

 tion — is the theory of Avogadro and Ampere, Koenig and 

 Clausius, on the constitution of perfect gases, to which 

 chemists, unknown to themselves, have been led in the 

 progress of their science. He thought the time had 

 arrived for reversing the order which had hitherto been 

 followed in teaching chemistry, that instead of setting 

 out from the criteria for determining the weight of mole- 



cules, and then showing their ratio to the vapour densities, 

 they ought, on the contrary, to commence with t e latter, 

 with the theory of Avogadro and Clausiu-;, demonstrating 

 it from physical considerations ; to found upon that the 

 proof of the divisibility of simple bodies, that is to say, 

 the existence of atoms ; and to show, as occasion 

 presented itself, that the weights of the molecules 

 and the number of the atoms deduced by the 

 application of this theory, are in accordance with 

 those which are deduced from chemical criteria. By this 

 means we can measure the degree of confidence to be 

 placed in the latter criteria ; since so-called compound 

 equivalents do not suffice to determine the weight of 

 molecules, or even to prove their existence, although they 

 may be deduced from a single principle, the theory of the 

 constitution of gases. This is the natural transition from 

 physics to chemistry. 



The Professor then stated in detail how he applied the 

 principles he had laid before them. He introduced his 

 pupils to the study of chemistry, by endeavouring to place 

 them on the same level as the contemporaries of Lavoisier 

 and to teach them to appreciate the importance of the 

 principle of the conservation of the weight of matter 

 showing them that this is quite independent of any idea 

 of_ its nature or constitution. They are thus led to e.xa- 

 mine the ponderable composition of substances, so that 

 the student passes rapidly from the epoch of Lavoisier to 

 that of Proust, and then to that of Berzelius at the time 

 when he commenced his researches on proportions. At 

 this stage the same impulse is given to the pupil as 

 Berzelius received on becoming acquainted with the hypo- 

 thesis of Dalton. The latter is laid before him without 

 any accessory, the use of symbols and formula; being in- 

 troduced dogmatically. There will now arise in his mind 

 the same doubts and difficulties that assailed BerthoUet, 

 Sir Humphrey Davy, and Wollaston in the application of 

 Dalton's theory, and at the same tim= a desire for an ex- 

 planation of the simple relation which exists between the 

 vapour volumes of bodies which react on one another and 

 of the products which are obtained. Now is the moment 

 to state or recall to mind the physical theoiy of th^ con- 

 stitution of the perfect gases. Commencing with a rapid 

 glance at their general and special characters he in ;isted, 

 that in this part of the instruction the mind of the student 

 should not be diverted from the numbers expressing their 

 relations, by considerations of the variations caused by 

 changes of temperature and pressure. In applying the 

 theory of the constitution of gases, it will be perceived 

 that the molecules of simple bodies are not always 

 the atoms of Dalton, and a certain confusion will 

 thus be produced in the mind of the beginner in the 

 conception of the ideas of atoms and molecules. The 

 hypothesis of Dalton can now be laid aside, sub- 

 stituting as a starting-point the theory of the rela- 

 tion of molecular weights to the vapour densities. A 

 table must be prepared of the vapour density compared 

 with that of hydrogen as 2, that is to say, the weights 

 of their molecules compared with the weight of the semi- 

 mdecule of hydrogen taken as unity. We must then 

 compare the composition of the molecules containing the 

 same element — including, or not, the molecule of the ele- 

 ment itself — and thence deduce the law of the existence 

 of atoms, that is to say, the amount of each element which 

 always enters, by whole multiples, into the molecules 

 which contain them. We here have the atoms of Dalton 

 which, in the present state of the science, express not 

 only all that Dalton discovered, but also the composition 

 of equal volumes of their vapours, and in the choice of 

 which those doubts can no longer arise which embarrassed 

 Davy and Wollaston. The ideas of molfcules and atoms 

 suggested to the student by this law a e d-voui of all con- 

 siderations of form, size, continuity, or discontinuity; the 

 only property indissolubly connected with them is that of 

 ponderability ; the very definition of matter. 



