beer yeasts only being exceptions with regard to the latter test. It has lately been shown that the blue colouration produced in guaiacum tincture is due to the action of a separate enzyme, belonging to the class of oxydases, and Loew has still more recently made experiments showing that the power of decomposing hydrogen peroxide is due to the action of another separate enzyme of general occurrence, termed by him catalase.

The changes which the carbohydrates undergo are by far the most important, on account of the fact that the products obtained from them have been the substances upon which most investigation and research have been expended, and many of them are of commercial importance, e.g. the production of alcohol from starch. It will, therefore, be more advantageous to describe firstly the changes which these undergo.

The carbohydrates are generally divided into four classes: (1) the Polysaccharides; (2) the Trisaccharides, or Saccharotrioses; (3) the Disaccharides, or Saccharobioses; (4) the Monosaccharides, or Monoses.

To the monosaccharides, the glucosides, most of which are derivatives of glucose, are very closely allied, and the chemical changes which these undergo as the result of fermentation are best considered before those which take place in the monosaccharides, especially glucose, and which result in the formation of products other than alcohol, e.g. the production of lactic acid. Hydrolytic changes also occur in the fats and oils, and in urea, and these are best reviewed before the changes caused by oxidation, the most important of which is the production of acetic acid from alcohol; we shall then come to

reduction-changes and the processes of nitrification and denitrification, and finally to the changes occurring in the albumins, which are of a much more complicated nature, but no whit less interesting or important than those occurring in the carbohydrates.

CHAPTER I.

CHANGES IN THE POLYSACCHARIDES.

Of the polysaccharides, starch is by far the most important, not only on account of the great rôle which it plays in nature as a reserve material in plants, but also on account of the numerous investigations which have been carried out by various observers, in order to elucidate the chemical changes which it undergoes in plants as the result of the action of the ferment, or enzyme, known as diastase, upon it. These investigations have been made with the ultimate view of determining its constitution, as well as the constitution of the several decomposition products obtained from it during the process.

As is well known, a thin starch paste, containing about one or two per cent. of starch, to which a little diastase has been added, gradually becomes clear and the starch dissolves. This solution now is found to possess the power of reducing Fehling's solution, a property which it did not possess before the addition of the diastase, and, if it be tested at intervals, during the process of this transformation, with a solution of iodine in potassium iodide, a series of colour-changes will be observed at first a blue colour is formed, showing still the presence of starch, but quickly this colour changes to purple, and then gradually to reddish-brown, until finally no colour-change

at all is seen. The purple colour is due to a mixture of starch and dextrin, the reddish-brown to a dextrin, and the cessation of colour-change indicates the presence of another dextrin and maltose. These changes, then, are the result of the hydrolysis of the starch through a series of dextrins to a reducing sugar, now known to be maltose.

Payen and Persoz, who, in 1833, first prepared diastase, were the first to examine the products resulting from this change in starch, and they found that a reducing sugar was not the only product obtained, but that a dextrin, not coloured by iodine and soluble in dilute alcohol, was also formed.

In 1860 Musculus observed that the dextrin and the sugar were formed simultaneously, and that the dextrin was not further altered. Independently of each other, about ten years later, Griessmayer, O'Sullivan, and Brücke ascertained that at least two dextrins were formed in this change, one of which was coloured brown by iodine, whilst the other was not visibly affected. Brücke named these respectively erythrodextrin and achroodextrin. O'Sullivan considered them to be identical, and said that they were convertible into maltose by malt extract.

Further investigations were made by Musculus and Gruber in 1878, who came to the conclusion that at least three achroodextrins were produced, the starch molecule breaking down by a series of hydrations and decompositions, maltose being obtained at each stage, together with a dextrin of decreasing, or lower, molecular weight.

In 1879 Brown and Heron published the results of the work which they had done upon the decomposition of starch by the action of diastase. Their conclusions, which

do not differ on the whole from the theories put forward by Musculus and Gruber in the previous year, are to the following effect: Starch has a formula of at least 10C12H20010; in the hydrolysis by diastase, groups of C12H20O10 are gradually split off, a dextrin of lower molecular weight being left at each stage, and eight of these dextrins are possible; the group of C12H20010, which is split off, is hydrolysed to maltose, C12H22O11, the elements of a molecule of water being taken up.

In the same year O'Sullivan stated that four dextrins were formed, one of them being erythrodextrin and the other three achroodextrins.

About the same time Herzfeld brought forward still another view of the changes produced in starch, as the result of his own experiments. He observed that the products obtained were different when the hydrolysis was carried out below or above 65° C.; at the lower temperature maltose and achroodextrin were formed, and the latter was subsequently converted into a body, which he called maltodextrin, and maltose; at the higher temperature, in addition to these products, dextrin and erythrodextrin were formed. He concluded, therefore, that there was consecutively a conversion of the starch into soluble starch, erythrodextrin, and achroodextrin, and that the latter was then transformed into maltodextrin and maltose. For maltodextrin he suggested the composition of two dextrin groups united to a sugar group of the C6H10O5

formula C6H12O6; thus: C6H1005

C6H12O6

In 1885 the results of a very thorough investigation