Critical point drying is a standard method for preparing SEM samples. Specimens are dehydrated in a solvent such as ethanol, then placed in the CPD where the solvent is exchanged with liquid carbon dioxide. Raising the temperature of the sealed "bomb" raises the liquid carbon dioxide past the critical point, where the density of the liquid phase is equal to that of the vapor phase. This process avoids the surface tension effects of normal drying and so preserves the natural structure of the sample.