the extinction, the linear and circular polarization, and the scattering of nebular 

 and star light. Interstellar extinction and polarization curves have been deter- 

 mined for numerous stars with diffuse interstellar material along the line of 

 sight. The observed extinction and polarization curves can be used to determine 

 grain sizes if their composition (i.e., optical constants) is known. However, very 

 little information of this sort is available for dust in molecular clouds. 



The dust can also be studied through its interaction with the gas, although it 

 is difficult to extract information on the dust from these data. For example, the 

 dust may significantly influence the molecular composition of the gas, but 

 because of the difficulty in disentangling gas-phase and grain-surface reactions 

 only H 2 is unambiguously attributed to grain-surface reactions. 



Finally, the dust can be studied indirectly by determining the depletion of 

 the elements in the gas phase with respect to solar abundances. In such an 

 approach it is generally assumed that the missing elements are locked in solid 

 dust grains. Correlations between the observed depletions and physical param- 

 eters, such as condensation temperature, may then lead to insight on the specific 

 composition and condensation history of the dust grains. 



Despite 50 years of active research, the composition of interstellar grains is 

 still highly uncertain. Most researchers in this field agree that silicates and some 

 form of carbon are present. The detection of the 10- and 20-jum features (pre- 

 sumably the Si-0 stretch and O-Si-0 bend in silicate materials) in a large variety 

 of objects points toward the ubiquitous presence of silicates in the interstellar 

 medium. The composition of the carbonaceous component is, however, more 

 controversial. Some researchers think that the carbon is mainly in one or another 

 highly condensed form, based on the strong 2200-A peak in the interstellar 

 extinction curve, which can be attributed to ~200-A nearly spherical carbona- 

 ceous grains. In such a model it is assumed that larger (~1000-A) carbon grains 

 are also present, which then produce a large fraction of the visible extinction. A 

 small fraction of the visible and the far-ultraviolet extinction, and the visible 

 polarization, is produced by silicate grains with sizes in the range of 100 to 

 2500 A. Inside molecular clouds these grains can accrete molecular mantles 

 consisting of simple molecules such as water, ammonia, and carbon monoxide, 

 plus all the other possible condensable interstellar molecules. But in the diffuse 

 interstellar medium these grain mantles are presumably quickly removed by 

 photodesorption and sputtering in low-velocity shocks. Good observational 

 evidence exists for the presence of icy grain mantles inside molecular clouds and 

 for the absence of water ice grain mantles in the diffuse interstellar medium. 



In an alternative model, the carbon is mainly in the form of large molecules 

 in grain mantles on silicate cores. Some carbon (~25%) is still in some highly 

 condensed form, in order to explain the 2200-A peak. The bulk of the visible 

 extinction and polarization in the diffuse interstellar medium, however, is due to 

 these grain mantles. The molecular mantles are presumed to be formed by ultra- 

 violet photolysis of the simple molecular mixtures accreted on the silicate grains 



