ISogal Jfttgtitution of ©reat 13rttam, 



WEEKLY EVENING MEETING, 



Friday, January 16th, 1885. 



The Duke of Northumbebland, D.C.L. LL.D. President, in the 



Chair. 



Professor Tyndall, D.C.L. LL.D. F.R.S. M.B.I. 



On Living Contagia. 



By desire of our excellent Honorary Secretary, Sir William Bowman, 

 to all of whose requests I wish to pay dutiful attention, I have 

 incurred the risk of standing before you here to-night. I speak thus, 

 because the time at my disposal, since the conclusion of the Christmas 

 lectures, has been far from adequate to the preparation of this 

 discourse. Its subject will, in the main, be a review of the labours of 

 Pasteur. Before he was born, Arago and Biot had discovered that 

 rock crystal, cut in a certain direction, possessed the extraordinary 

 power of rotating the plane of polarized light. Some samples of the 

 crystal turned the plane of polarization to the right, and others to the 

 left ; they were therefore called respectively, right-handed and left- 

 handed crystals. The power disappeared when the crystal was 

 dissolved, and did not therefore reside in the molecule or unit-brick 

 of the crystal. Biot afterwards discovered that many liquids possessed 

 this power of rotation ; and in these cases the rotary force must have 

 resided in the molecule. Two compounds of tartaric acid had been 

 discovered which, in solution, turned the plane of polarization, the 

 one to the right, and the other to the left. The left-handed tartrate 

 was discovered by Pasteur. Prompted by an observation made in 

 Germany, Pasteur mixed the pure, right-handed, tartrate of ammonia 

 with some albumenoid substances, and exposing the mixture to a 

 gentle heat, found that it fermented. The solution, at first limpid, 

 became turbid, and this turbidity was proved to be due to the multipli- 

 cation in the liquid of a minute microscopic fungus. A solution was 

 then prepared, wherein the right-handed and left-handed tartrates 

 were so equally balanced that they completely neutralized each other. 

 The solution had at first no power over polarized light. But 

 immediately after fermentation had begun, or in other words, after 

 the fungus had begun to multiply, a rotation to the left was observed. 

 This increased until it reached a maximum, when it was found that 

 all the right-handed tartrate had disappeared from the liquid, leaving 

 the left-handed tartrate behind. Now the two tartrates were alike in 

 chemical composition ; they possessed the same atoms and the same 

 proportion of atoms ; but owing to a difference in the structure of 

 their molecules, one of them turned the plane of polarization to the 

 Vol. XI. (No. 79.) m 



