MEASURING VISCOSITY OF SEMISOLIDS 247 It should be noted that only the first two terms in the series are used for integration purposes. It can be shown that an error of less than 1% is introduced when the height- to-diameter ratio (H/2r) of the sample is within 0.60 and less than 2% error when the ratio is within 0.75. INCOMPRESSIBILITY OF MATERIAL Soft poly. meric solids have a bulk modulus of around 3 x 10 •ø,, dynes/cm 2 (16,17), indicating that such materials would require almost a ton (1000 kg) of pressure to cause as little as 1% volumetric change. Since the proposed method is an open system re- quiring the equivalent of approximately 100 g of pressure for necessary deformation, volumetric change was neglected in the above derivation. MATERIALS AND METHODS ISOTROPICITY OF MATERIAL Small quantities of PEG 1500 (Lot #1305, Ruger Chemical Co., Inc., NJ) and petro- latum (Lot #710683, Ruger Chemical Co., Inc., NJ) were melted and congealed for examination under the polarizing microscope (Nikon Microflex EFM with photomicro- graphic attachment, imposed with polarizer and analyzer, Nippon, Kogaky, Tokyo, Japan (Figure 4). These materials were found to be crystalline and hence anisotropic.* .On this basis some deviation from ideal behavior was expected since the use of tensor :analysis requires the material to be isotropic. APPARATUS ß A complete description of the main apparatus has been presented earlier (1). It consists of two horizontal parallel plates pressing the test material with a known stress, analo- gous to that reported for the parallel plate plastometer technique (12). A guide rod and a guide plate together with the use of three equal-sized sample plugs ensured the smooth descent of the top plate during measurements. Several substitutes of bottom plates were kept ready for quick changes. Teflon © was used to minimize friction be- tween moving parts of the device. The entire unit, which includes the main apparatus, an LVDT unit (linear variable differential transformer Schaevitz Eng., Type 1000 HR, S/N 5519, CAS Series, Pennsauken, NJ), a chart recorder (Sargent-Welch, Model SRG), and a Brookfield helipath stand, is shown in Figure 5. The applied stress is represented by the total weight of the top plate with guide rod, plunger of the LVDT unit, and any added weights. METHOD With the aid of an open-end 5-ml disposable Monoject syringe, the samples were drawn from the test material, previously melted, and set in preparation trays. A column of * We agree with the reviewer's comment that "The existence of anisotropic materials dispersed in an isotropic phase does not necessarily make the material behave in an anisotropic manner." However, whether the material could still behave as truly isotropic is also not certain.

248 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS :'..• ..•.•. '•f•'•'g? .'wx: 5.. 5•.. .... a"•:'•. •-.'.•37•"3.1•.. •" ? :?•evv. .. ,, • .... •.. •.. ½:. ß :..½.. •a... •. • • :.• "• Figure 4. Polarized photomicrograph of PEG 1500 (top) and petrolatum (bottom). Magnification 100 X