A text-book of the science and art of bread-making : including the chemistry and analytic and practical testing of wheat, flour, and other materials employed in baking / by William Jago.

  • William Jago
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of the science and art of bread-making : including the chemistry and analytic and practical testing of wheat, flour, and other materials employed in baking / by William Jago. Source: Wellcome Collection.

    62/740 page 30
    :^0 50. Ozone, O.., •—This body is <1 f^aseous substance consisting of ]>ure oxygen, l)ut having a density of I'd instead of IG. This is due to tliere being .G atoms of tlie element in the molecule, instead of 2 as in ordinary oxygen. (Jzono has a j)eculiar odour ; and is jiroduced during the working of a frictional electric machine, when its smell is recognisecl. Traces of this gas exist in the air in mountainous districts, and liy the .sea side. By exposure to a temperature of 2.‘57° C. ozone is transformed into ordinary oxygen. Ozone is a powei’ful oxidising agent, and is iinmical to the growth and development of germ life. 51. Water, H2O .—This most important compound consists of two volumes of hydrogen united to one volume of oxygen, to form two volumes of water-gas or steam. By weight, water contains IG parts of oxygen to 2 of hydrogen. Water in the pure state is odourless and tasteless; viewed through thick layers it has a blue colour. At tem¬ peratures below 0° C. water exists in the solid state; on being heated, ice expands until a temperature of 0“ C. is leached. At this point the ice begins to melt; the tempei’ature remains stationary until the whole of the ice is melted, but in order to effect the change from the solid to the liquid condition as much heat is required as would be .sufficient to raise 79 times the weight of water from U° to 1° C. Ice in melting contracts in bulk ; 10'9 volumes of ice producing 10 volumes of water. As the ice cold water is further heated, contraction continues until a temperature of 4° C. is I’cached ; at this point water is at its maximum density, and any given weight of it occupies its minimum volume. With further apjilication of heat the water expand.s, and also rises steadily in temperature. In metal ves.sels open to the air, water boils at a temperature of 100° C. Continued heating now converts the whole of the water into steam, but does not rai.se the temperature. The (juantity of heat necessary to convert the whole of the water at 100° C. into steam at the same temperature would i-aise i'^'M '2 times the weight of water from 0° to 1° C. kSteain in being further heated expands, and may have its temperature raised indefinitely : steam follows the same hiw of exiiansion on increase of temperature as do other gases. Steam, on being cooled, passes through a .series of changes which are the <‘xact converse of those just desciibed. At all temperatures water gives off vapour, but with much greater rapidity as the tempei'ature ap¬ proaches the boiling i)oint. This vapour exei'ts a (h'finite i)ressure, the jiressure increasing steadily with the tem})erature ; at the lioiling point, the pressure exerted by the vajtoui’ of water is exactly e(]ual to that of the atmosphere; cons(‘iiu<mtly, if the atmospheric pre.ssure lie diminished, the boiling jioint of water, and also that of all otlnn- licpiid.s, is lowered. Advantage is taken of this jiroperty in many o])t>rafions i?i the arts; thu.s, in driving oil' the wati'r from sugar solutions, as in the jirc'pai'ation of malt extract, the boiling is ( li'ected in a vacuum, and so the temperature prevented from rising to any great height. On the othi'r hand, by subjecting water to pressure-, its boiling jioint might In- rai.sed to any temperature attainabh-, the only limit being the caiiacity for n-sisting tlie pressure of the material of the ve-ssel. The tube-s of sti-am ovens aree constructed on this ])rinci})le—a c('i-tain (ptantity of wati-r is scaled up in them, which, on being heated, is converted into steam. Inn ing a
    digitised image 1
    digitised image 2
    digitised image 3