4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS in a very high state of purity, to mixtures of doubtful chemistry and very varied composition. They include aldehydes which are not aldehydes, Geraniol which is not Geraniol, Ionone 100 per cent which is far from 100 per cent, and a large number of other substances of undefined composition. Nevertheless, the perfumer is in no doubt when he receives one of these products as to its usefulness for his particular purpose. After all, a rose by any other name, etc. Hundreds of organic compounds have been identified among the naturally occurring essential oils and to these have been added many hundreds of synthetic chemicals to form the group which we now term "perfumery chemicals". The perfumer's interest in this large number of compounds can be split into three sections: trirstly, he requires stable compounds of constant quality which are pleasant smelling and of moderate price. These are of prime importance and form the backbone of his perfume. Secondly, he is interested in those products which have specific or unusual odours so that he can formulate perfumes with different characteristic notes. Perfect stability and moderate price may not in this case be of first importance. Thirdly, he is greatly interested in low priced products having a pleasant odour and good stability which can be used in deodorants and other industrial perfumes. Here, cost may be his main consideration. The first of these three sections forms the main subject of this talk, but the observations apply also to the other two sections. By long use and custom, the trade identifies a particular chemical by a particular odour, but it is known that this odour can be considerably modified by the presence of subsidiary constituents. This is especially so with the isolates. It is fully appreciated that when a successful perfume has been known to the public over a period of years, it would be disastrous for a manufacturer suddenly to compound this perfume wholly or partially from pure products the odour of the resulting perfume would, in most cases, be drastically altered. Despite these statements, there is a need for perfumery chemicals of improved quality, and considerable advantage could be obtained by the use of chemi- cals of a relatively high degree of purity. In the first place, there is advantage to the perfumer who wants, above all things, reproducibility of odour. Once he has decided upon his odour standard for a given chemical, then each succeeding delivery must be reason- ably close to that standard. The nearer the approach to one constant odour, the easier it becomes to repeat that odour effect in subsequent batches. Impurities, as distinct from subsidiary constituents, are by-products or residuals which remain in the product during its manufacture, and it is frequently difficult to ensure that the same amount and the same kind of

PERFUMERY CttEMICAI. S 5 impurity will be left in every batch. Naturally, the smaller the amount of impurity present, the less a change in amount will affect the final odour. Secondly, a higher degree of purity is of great assistance in laboratory control of quality, as variations in trace impurities with strong odours may have little or no effect on the physical or chemical constants, but a very great effect on odour. This places an undue strain and responsibility on the perfumer, who must decide whether a product is satisfactory and similar to that previously supplied. It is possible to produce synthetic chemicals in a very pure state, and these obviously suffer less from variations in quality, but whether they will be useful as perfumery materials is open to doubt. THE •/[ODERN TREND To sum up the situation, it appears that the modern trend is to use materials of the highest possible degree of purity. Although there are reasons why this is a most desirable trend, there will be many cases where the pure product will be of little value to the perfumer. The question is, how can these opposing points of view be reconciled ? The first approach is by a better understanding of the nature of a perfumery chemical and an apprecia- tion of what the perfumer really expects of that particular chemical by way of odour performance. There is no doubt that the modern methods of analysis are helping us to appreciate the foregoing points. When a perfumer feels there is a desire for a superior grade of perfumery chemical, then he must distinguish between unwanted, unpleasant impurities and other subsidiary constituents. The subsidiary constituents referred to fall into three classes: 1. Those which have little or no effect on the odour. 2. Those which are essential to the odour of the main chemical to impart life or fullness and roundness. 3. Those products where the minor constituent is almost entirely responsible for the odour of the named substance. These cases are by no means rare. The present position has come about by the fact that with the older type of plant it was impossible to separate traces of undesirable impurities from the subsidiary constituents. The result was that the subsidiary constituent was often unjustly blamed for the unpleasant odour. Using modern tech- niques, it has been shown that in many cases high proportions of subsidiary constituents actually fortify the main odour and are absolutely essential, always providing that these constituents are free from unpleasant trace compounds. PRACTICAL EXAMPLES As a practical illustration of the points so far outlined, I should like to take as an example one of the extreme cases, namely, Rhodinol. Consider