



in ■which, hy means of the force derived from the rays of the sun, all these different substances, are 

 analvzed and re-combined into very complex organic materials. 



The plant with which we have been experimenting Las green leaves, and has been able to break up 

 the carbonic acid of the air, and make use of the carbon to build up its substance. 



It is a strange fact that the presence of a green colouring matter in plants should be accompanied 

 by this power of making use of carbonic acid, but there are many living beings belonging to the plant 

 world which do not possess any green colouring matter in their tissues, and are therefore obliged to 

 provide their carbon from compounds into which it has already been worked up by green plants. 



Amongst the flowering plants there are a few destitute of green, but the fungi are universally so. 

 In oider to obtain their carbonaceous food, they live eilher on decaying organic matter, or they attack 

 living tissues. Many of them simply extract the food they require ; but others set up a decomposition 

 in the materials upon which they are feeding, without making any use of the products of this decom- 

 position. It is this kind of action, again, which is called fermentation. 



Yeast is one of these iungus-ferments. If a drop of the frothy 6cum is examined under a micro- 

 scope, enormous numbers of round bodies will be seen floaling in the fluid Each one consists cf pro- 

 toplasm, the bitsis of all living matter, and this substance is enclosed by a thin transparent covering- 

 of cellulose. These round bodies are so minute that if 1.000 of them could be placed in a line, they 

 would only measuie one-third of an inch. Yet euch is a self -contained living being, and if put into a 

 suitable nutritive fluid will multiply to such an enormous extent that it has been calculated that one 

 yeast cell can in 24 hours produce 1G million cells. The commercial impoitance pf yeast is due to ihe 

 transformation which sugar undergoes when 6plit up to afford it food. Sugar materials aie divisible 

 into two groups, one represented by saccharose or cane sugar, the other by glucose. They differ in 

 their chemical formulae, cane-sugar having a molecule less ef water than glucose Befpre cane sugar 

 can be fermeuted, it must be changed into glucose, and this is effected by a diastase founcd by the 

 yeast itself, though the re-action also takes place under the influence of dilute acid. In the oidinary 

 fermentation of malr, Pasteur showed that about 90 per cent, of the sugar was convcrled into alcohol 

 and carbonic acid, about 5 per cent, into glyceiine and succinic acid, and the remainder was used for 

 food for the yiast-plant to supply suLstai.ce for the cellulose envelope and the fatty partich s in the 

 protoplasm. The proportion ot glycerine tnd succinic acid is greater when the process of fei men- 

 tation is lengthened, end is less when the liquid is slightly acid. 



Pasteur placed different yeast plants in identical liquids, and showed that beers of diffeient flavouia 

 are produced, which possibly may be explained by supposing that each one has its own particular fer- 

 mentative action, producing bye-products which vary in quality and quantity. 



Il.c gieat disproportion between the amount ot sugar broken up into alcohol and the amount actual- 

 ly med as food by the yeast plant is very striking, and seeing that there is gieat expenditure in the pro- 

 duction of alcohol which is positively hurtful to the yeast, stopping its action after a time, the question 

 arises whether tiiis development of yeast with so great a formaiion of alcohol is the noimal com: i ion of 

 the pi int. In alcoholic fermentation, very little air is allowed to the fe rmenting liquid, and it is f< ur.d 

 that a very thin layer of the liqui i with fiee access of air allows the yeast to develop rapidly with very 

 little forma 1 . ion of alcohol. 



Pasteur pioved this conclusively. First, he placed yeast in a sugary liquid which covered the bot- 

 tom, of a large vat in a thin hnev, so that the air would have free access to the whole. The p] oration 

 was stepped when a layer of \east was deposited at the bottom of the vat. He founel that the develop- 

 ment of the yeast had been as abundant as it could be, one part to four of the sugar, which had disap- 

 peared. The production of alcohol was insignificant, the greater part of the sugar having u n used up 

 to ioren the ternary compounds of the yeast. As a second experiment, he took a flask full of a solution 

 of sugar fiom which air hud been < xpelled by boiling, and put a particle of yeast into it. When the fjr- 

 mentaiion was complete he found that very little yeast had teei poduced, in about the proportiou of 

 one part to 90 of the sugar which had been used up, but that almost the whole of the sugar had undergone 

 a reguLr alcoholic lei mentation. 



M iiu faeturcis of yeast as a commercial product car so arrange their processes that y.?a«t multiplies 

 without a trace ot alcohol. Certain common moulds which prow freely on any damp surface, do not pro- 

 duce alcohol under oidinary circumstances, but if they are immersed in a pie>pcr liepuid, they will excite 

 alcoholic fermentation to a small extent. These considerations lead us to suppose that alcoholic fomen- 

 tation is not the noimal aithn of healthy veast, but is due to a diseased condition of the cell dc^endeat 

 on the exclusion of a certain amount of air. 



The piaeiical co: clmi n is thai while aceitain araountof oxygen is as necessary to they east-plant as 

 to higher nem^s, giving it energy, tnd iueiueing a e]uioker fermentation, an excess will lead to a great 

 waste of sugar. 



Other ferments also pioduce alcohol, and the cells of ripening fruit lose a poition of their sugar with 

 iermentation of alcohol wuai the access of air is cut off 



bo full was Pasieur < i the idea that the cells of a fiuit would continue to live at theexpense of the 

 augar of the fiuit, that once in his laboratoiy, while conveising on these subjects with M. Dum is, he ex- 

 claimed 'I will wager that if a giape be plunged in an atmosphere of caibenic acid, it will produce 

 .alcohol and carbonic acid by the continued life of its own cells — and they will act for a time like the cells 

 of ihetiue alcoholic haver..' lie mtde the expeiiment. and found the result to be what he hud fore- 

 seen. He then extended the inquiry. Placing under a bell jar 24 plums, he filled the jar with eai tonic 

 acid gas ; beside it, he placed 24 similar plums uncovered. At the end of eight daj s, he removed the 

 plums from the jar and compared them with the others. The difference was extraordinary. The un- 

 coveied frails had Leceme soft, watery and very 6wcet ; the otheis weie fii m and hard, their fleshy por- 

 tions beinp not at all watery. They had, moreover, lost a considerable quantity of their sugar. They 

 wue afterwards bruised and the juice was distilled. It yielded six and a half grammes of alcohol, or one 



