SCIENCE. 



93 



tone in the liquid, being one-half of the dry weight of the 

 peptone. The hydrocarbons should, however, always be 

 only from . 5 to I per cent, of the weight of the entire 

 liquid, and should even then serve exclusively for the for- 

 mation of the walls of the cells of the yeast. 



The vegetation of the yeast will take place most satis- 

 factorily at temparatures varying fiom 57 to 64 degrees 

 Fahrenheit. At a higher temperature losses may easily 

 occur by reason of the partial conversion of the sugar 

 used into coagulated acid or into alcoholic fermentation, 

 instead of furnishing the yeast with substance for cells. 

 The yeast is either propagated, as is the custom in Hol- 

 land, in shallow vessels in which the depth of liquid is 

 about five inches, so that a sufficient quantity of atmos- 

 pheric air has access thereto ; or it may be better and more 

 safely effected in vats made of wood, glass, masonry, 

 cement, or other suitable material, into which atmospheric 

 air is conducted by suitable dis'ributors through tubes or 

 pipes by means of blowers or compressers. 



Instead of atmospheric air alone it is more advantageous 

 to use air containing an increased amount of ozone or of 

 oxygen partially converted into ozone. The latter is pre- 

 pared by successively adding hydrogen dioxide to the 

 propagated liquid. The percentage of ozone in the air is 

 increased by means of phosphorus, or by causing it to 

 pass through a closed vessel in which permanganate of 

 potassa is mixed with the necessary quantity of mineral 

 acid. The air thus enriched with ozone is then allowed 

 to pass into the propagating liquid. 



The growth of the yeast will be completed within 

 from 6 to 8 hours after every sufficient addition of dex- 

 trose, maltose, or other material, according to the density of 

 the propagating liquid used, the temperature of the latter, 

 and the amount of the ozone in the air. The percentage 

 of peptone of the mass may amount to from 1 to 2 per 

 cent, or more of its weight, while only from one-half to 

 one per cent, of dextrose or other hydrocarbons is added 

 at each time, in order to be sure to prevent the formation 

 or coagulated lactic acid or alcoholic fermentation. 



When the entire amount or bulk of the dextrose or 

 other sugar added to promote the growth of the yeast 

 has been consumed after from six to eight hours, a further 

 quantity thereof, say, from .05 to .10 per cent, is added. 

 The peptone may also, after having been consumed, be 

 added in portions, or may be allowed to flow in gradu- 

 ally and continuously. The same propagating liquid 

 made by successive replacement of the matter consumed 

 remains in use for weeks or months, unless it is rendered 

 impure by other substances, or by subsiding fermentation 

 is made unfit for further use. In the same manner as 

 the materials necessary for the propogation of the yeast 

 are added the yeast produced may be successively with- 

 drawn, and only the yeast suspended in the liquid re- 

 mains behind as the germ for the ferments of alcohol to 

 be afterwards formed. The yeast is obtained either by 

 ski mming it from the surface of the liquid or by separat- 

 ing it from the propagating liquid by filtration, or finally 

 by gathering it after tapping the vats from the bot'om 

 upon which it is deposited in a compact layer. In work- 

 ing on a large scale it is advisable to place the vats in 

 terraced batteries in order to effect the transfer of the 

 propagating liquid from one vessel to the other with 

 facility. In order to produce yeast as free as possible 

 from subsidiary ferments the propagating liquid may be 

 prepared in a more dilute state, that is to say, with a 

 percentage of peptone of only from .75 to 1 percent. 

 The hydrocarbons (dextrose, maltose, or the like) may 

 also be added in smaller quantities, for example, as a 

 first dose about .33 per cent, and then every 3 hours 

 about .05 per cent. 



The greater part of the peptone present will then be 

 transformed into yeast in from 12 to 15 houis, a sufficient 

 supply of pure air, if necessary, conducted through 

 sulphuric acid or oxygen containing ozone, being pro- 

 vided, and the entire process being carried on at a tem- 



perature varying from 54 to 63 degrees Fahrenheit. The 

 whole liquid is then cooled by a suitable apparatus, or 

 by adding cold water or ice ; the best temperature being 

 from 45 to 50 degrees Fahrenheit. Within from 36 to 

 48 hours the yeast obtained will settle on the bottom of 

 the vat. The propagating liquid may be allowed to flow 

 away. The yeast obtained by this improved process is 

 purified and condensed in the usual manner, but in order 

 to increase its durability phosphate of lime amounting 

 to from 4 to 5 per cent, of the total weight of the yeast 

 to be made may be added before compressing it. 



Experience has shown that from 250 to 300 parts of 

 pure and active compressed yeast may be obtained from 

 100 parts of pure peptone. For the growth of that 

 quantity of yeast only about 200 parts of dextrose or 

 sugar are required. 



MICROSCOPY. 



We have received the February issue of the Journal of 

 the Royal Microscopical Society, now edited by Mr. Frank 

 Crisp, one of the secretaries of the society. It contains a 

 valuable and interesting original paper, with two full- 

 page illustrations, and the proceedings of the R. M. C. 

 A summary is also presented of current research in those 

 departments of science, depending upon the use of the 

 microscope for their advancement. The amount of infor- 

 mation thus gathered may be estimated from the fact 

 that the present number is a volume of one hundred 

 and seventy-two pages. The Journal appears bi-monthly, 

 and costs one dollar (4s.) for each part. 



The President of the Royal Microscopical Society an- 

 nounced that a fund had been provided for the presentation 

 of two gold medals annually, without regard to nation- 

 ality — one for the person who should originate any im- 

 portant improvement in the microscope, or any of its ac- 

 cessory apparatus, or in any other way eminently contribute 

 to the advancement of the microscope as an instrument of 

 research. The second gold medal was to be awarded " in 

 respect to any researches in any subject of natural 

 science carried on wholly, or in a great part, by 

 means of the microscope, or of the recipient having in 

 other ways eminently contributed to the advancement of 

 research in natural science in connection with the micro- 

 scope." 



The two medals were to be known respectively as the 

 " Microscopical" and " Research " medals of the Society. 

 For reasons which are not stated, the offer of this fund 

 was declined by the Council of the Society. 



The war of Apertures of Microscope Objectives has 

 again broken out in the R. M. S. In this instance Mr. 

 Shadbolt was the aggressor, who claimed that his paper 

 demonstrated beyond dispute the following facts, viz.: 



" That a dry lens can have as large an ' angular aper- 

 ture ' as an immersion one, and that the assumed differ- 

 ence of aperture between dry and immersion lens does 

 exist." 



" That no lens can have an ' aperture ' of any kind 

 which exceeds that of 180 angular in air." 



" That, consequently, the table of ' numerical apertures' 

 published on the cover of the Journal of the Society is 

 erroneous and misleading, and should at once be dis- 

 continued." 



In reply, Mr. Crisp asserted that Mr. Shadbolt was in 

 error, and the victim to a misplaced confidence in a fun- 

 damental fallacy, viz., " the supposition that equal angles 

 in different media, as air and oil, are optically equivalent." 



A correspondent, who is an authority on this subject, 

 will offer an opinion on this matter. We believe, how- 

 ever, that Mr. Crisp is correct in his views, and that the 

 society has exercised a wise discretion in putting a stop 

 to a discussion, which had become wearisome and un- 

 profitable. 



Mr. Crisp showed how a few moss-grown English 

 microscopists had persistently refused to countenance 



