METHOD FOR DETERMINING SPRAY PATTERNS FROM PRESSURIZED PACKS 225 be changing while they are still close to the dispenser, due to the rapid evaporation of propellants and volatile solvents. The method is, therefore, primarily of interest where products intended for surface spraying are being examined, and these should be sprayed at the recommended distance from the plate. If used in this way the apparatus gives valuable results with a wide range of products, such as crawling insect sprays, perfumes, hair lacquers, sun tan oils and others applied to surfaces. It also gives useful comparisons of finer sprays. Due to the statistical nature of the droplet distribution absolute repeat ability of the patterns cannot be expected. None the less a satisfactory degree of duplication of the general type of pattern is obtained as is shown in Figs. 6a and 6b. The two different patterns were obtained from the same dispenser, which contained 20 per cent kerosene and 80 per cent 50/50 Arcton 6/9 mixture. ACKNOWLEDaEMENTS The author wishes to thank Cooper, McDougall & Robertson Ltd., for permission to publish this article, and Mr. R. M. Mumford for his help and advice. He also wishes to thank Mr. G. McGregor and Mr. P. E. Longhurst for their assistance in its development. [Received: l•tth October 1958• DISCUSSION DR. H. W. HIBBOTT: How much does distance and size of slot affect the pattern and the fact that propellant might be carried to the plate or evaporate before it reaches the plate ? Would it be more conclusive if the dispenser were so far away that all propellant will have evaporated before it reaches the plate ? THE LECTURER: We do not really know how long it takes for the propel- lant to evaporate. In the case of sprays with very fine particles one might well have to be so far away that the finer particles would not land on the plate at all. Most surface sprays are used in a manner which makes it very unlikely that all the propellant evaporates before it reaches the spray area. It is therefore better not to spray from too far a distance when recording spray patterns. MR. A. G. MCGEE: Have you any experience of recording spray patterns by using a coating of dyed talc on paper, and if so, do you consider your method to be more advantageous ? T•i• LECTURER: You are referring to Root's* method. I have found it difficult not only to get a uniform coating of dyed talc on to the paper but have also failed to achieve the standard of spray pattern shown by Root.* *Root, M. J. J. Soc. Cosmetic Chemists, 7 (1956) 153.

226 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I consider Root's method to be neither as reproducible nor his patterns as permanent as the results obtained by my method. MR. W. SCaON: Does the size of the hole in the magnesium oxide film vary with the speed of the particles ? THE LECTURER: I really do not know. It depends on droplet size and thickness of coating. If the thickness of the coating is greater than the droplet diameter one will get a hole somewhat larger than the droplet. If it is not, then I believe the droplet is likely to spread on impact and give a hole considerably larger in diameter. MR. A. FOSTER: There appears to be a considerable range in droplet size. Can the largest of these be taken as representation of single particles, or might they be due to coalescence ? TH• L•CTURF•R: If the hole is circular, it is almost certainly due to a single droplet. If it is of a different shape then it may be due to more than one droplet. The correct result can be achieved by interpreting the plate carefully, and the ability to do so grows with practice. DR. W. MITCHELL: As the particles of magnesium oxide are very fine and therefore presumably very reactive, is there a possibility that certain products might react with the magnesium oxide and that the holes produced are then larger than they should be ? Ta• L•CTURER: I have never found this to occur with cosmetic and insecticidal sprays. It may, however, occur with other products. COMPRESSED GASES AS PROPELLANTS FOR COSMETIC PRODUCTS A. HERZKA, B.Sc., A.R.I.C.* A paper delivered before the Society at the Symposium on Pressurized Packs on 14th October 1958. The dffierence between liquefied and compressed gases are described. Reference is also made to the quality factors which are of importance when formulating with compressed gases. NEARLY TWELVE months have elapsed since the publication of a feasible way of dispensing viscous products in non-aerated form from pressurized packsL It must not be assumed, however, that the idea of utilizing nitrogen and other compressed gases as propellants is a new one. Root• has ably reviewed * Research Department, The Metal Box Co. Ltd., Alperton, Middx.