The 21-cm evidence for an intercloud medium is reviewed. The observations include the sky distribution and velocity structure of 21-cm emission profiles, self-absorption features in emission profiles, and absorption profiles in the directions of discrete sources. It is concluded that the intercloud medium in the solar neighbourhood has a temperature between 103 and 104 K, a column density of ~1.4 × 1020 cosec |b| atoms cm-2, and a brightness temperature of ~4.4 cosec |b| K wherever the line of sight does not intersect optically thick cold concentrations.

We compare past and present modes of investigation of the structure of the interstellar gas. Many aspects of the interstellar cloud model are invalid.

Interstellar optical absorption lines and Hi21-cm emission lines show a number of very large aggregates with properties similar to those of ‘cloud complexes’. At nonzero velocities especially for b<0°, exist optical lines which have no Hicounterparts. These are almost certainly produced in low-density gas clouds; perhaps the intercloud medium is itself cloudy.

Maps of Hicolumn density taken over large velocity ranges do not reveal much small-scale structure. This fact cannot easily be reconciled with the statistical analyses of interstellar reddening. The maps do reveal large, coherent gas structures which are often filamentary in shape and at least sometimes aligned parallel to the interstellar magnetic field.

Maps of Hicolumn density over small velocity ranges show much small-scale structure, often filamentary in shape. The filaments are almost universally oriented parallel to the interstellar magnetic field and have Doppler velocity gradients along their lengths. In one area the geometry of the field and gas almost exclusively suggests Alfvén-type motions.

In the region +19° <b< +30°, 122° <l< 143°, 21-cm line profiles were analyzed into Gaussian components, and these components combined into clouds, by means of computerized procedures. Maps of some of the clouds so defined are presented, and their properties discussed. Many clouds are elongated and/or irregular in shape. Histograms of cloud parameters are presented and subjected to statistical analysis. There is evidence for tenuous clouds of large angular extent but very low column density (NH<1019 cm-2).

This paper reviews the recent observational results on the electron distribution outside the Hiiregions. The mean electron density in the solar neighbourhood is of the order of 0.03 cm-3, and does not seem to vary very much between the Hiclouds and the intercloud medium, or between the arms and the interarm region. The z thickness of the electron layer in the solar neighbourhood is found to be larger than or of the order of 0.8 kpc. No reliable figure of the electron distribution can be deduced for the moment in the inner parts of the Galaxy.

The Copernicus satellite now opens up the ultraviolet region for inspection, and the number of resonance lines which may be studied has increased from 6, seen in the visible, to more than 30. The distribution and properties of an important constituent of the interstellar gas, molecular hydrogen, can be studied in detail using this instrument. A more comprehensive picture may now be developed for element depletion factors, electron densities, and sources of ionization (UV photons, low energy cosmic and X-rays).

There has been considerable uncertainty in the nature of the regions from which carbon and narrow hydrogen lines have been observed. Specifically there are two different models. In model I the lines originate in low-density Hiregions that are basically atomic, and stimulated emission enhances the line intensity by an order of magnitude. In model II the lines originate in the outer layers of very dense molecular regions and are due primarily to spontaneous emission. The first model is generally accepted and used by the astronomical community. However, we believe that the second is correct.

This paper reports two new recombination-line results. The first is the detection of carbon line emission from the dark cloud near ϱ Ophiuchi, and the second discusses the origin of hydrogen recombination line emission associated with ionized gas outside known discrete continuum sources.

The recent high-sensitivity pulsar survey at Jodrell Bank has allowed a statistical study of more distant objects. The longitude distribution suggests that many of the pulsars observed have distances greater than 5 kpc, leading to an upper limit of about 0.03 cm-3 for the mean electron density. The electron density averaged over distances of a few hundred parsecs seems to be very constant. The width of the electron distribution in the z-direction appears to be greater than about 600 pc.

Ionization and heating mechanisms are reviewed; some of them apply to the interstellar medium as a whole, others only to localized regions. Cooling processes are briefly summarized. Results are given from a recent model calculation for an intercloud medium heated by X-rays.

Numerical calculations of the growth of thermal instabilities show that condensations tend to form with the proper sort of density contrast but the wrong shapes (and, to some extent, sizes) to qualify as the observed interstellar clouds analyzed by Heiles.

Expanding shells of neutral hydrogen have been found connected with the associations Per OB2 and Sco OB2. The shells also contain dust and molecules. A model of the spatial configuration and the kinematics of gas, dust and stars is discussed. The stars of the associations lie outside, close to the shells; gas and stars move outward from a common centre of expansion. If the shells are interpreted as old supernova remnants which have been strongly decelerated by the interstellar medium, ages of the order of 1 × 106 yr are obtained. A genetic relationship between stars and shells is suggested. The stars formed in the densest gas shell; afterwards they separated from the gas, as the shell was further slowed down by intervening interstellar matter. Limits are set on the time scale of star formation. The geometry and the motion of the shell at the moment of star formation account also for the kinematics of Per OB2 and Sco OB2.

A number of possible mechanisms for the formation of molecules in the interstellar medium are discussed. Plausible suggestions have been made for the production of most of the known diatomics, but there is little understanding of polyatomics so far.

Low-velocity OH emission lines at 1720 MHz have been investigated in the direction of W41, W43, W51, and 3C 353. The emission lines in the directions of W43 and W51 show no detectable circular polarization and are about 27 and 32 arcmin in size, respectively. The inferred peak brightness temperatures are 0.6 K for W43 and 2.0 K for W51. This emission may be similar to the nonthermal 1720-MHz emission in dust clouds.

The main investigations of the local magnetic field are reviewed and are found to contain some conflict in interpretation. At radio wavelengths, studies have been made using both the Faraday rotation of the polarized radiation from extragalactic sources and pulsars, and the polarization of the galactic background radiation. With the former type of observation, although more data are available for extragalactic sources, any interpretation may be complicated by the influence of distant field structure. The results are consistent with a large-scale field parallel to the galactic plane, with a field strength of about 2 µG, and which is directed towards l=90°. This field contains irregularities in direction and strength on a scale of about 100–200 pc. The polarization of galactic background radiation may yield the most detailed information about the local field structure – the results to date show loops of magnetic fields extending along the radio spurs.

The interpretation in terms of small-scale irregularities embedded in a large-scale field parallel to the galactic plane differs from that proposed to explain the optical polarization of starlight, in which a helical field configuration near the Sun was preferred to a more disordered pattern.

Observations have been made at 21 cm with a resolution of 11′ to look for fine structure in the polarization distribution. In the North Polar Spur, the angular scale of the polarization parameters varies with latitude. This is attributed to an increase in the irregularity of the magnetic field in the Spur with latitude.

The chemical composition of interstellar grains is derived here on the basis of (1) the cosmic abundance of the elements; (2) the wavelength dependence of extinction and polarization; (3) the average total extinction; (4) the ratio of polarization to extinction; (5) the predominantly dielectric character of grains in the visible spectral region; and (6) infrared spectral characteristics of grains. It is indicated that the major portion of the grains, by mass, consist of core-mantle particles in the 0.1-µm size range, whose cores consist largely of silicates and whose mantles are a solid mixture of O, C, and N with H in a heterogeneous combination of simple and complex molecules with frozen free radicals. A minor constituent of the solid particles exist in the form of very small uncoated particles generally less than 10-6 cm in size whose precise composition is not certain. Inferences of the core-mantle model with respect to spatial distribution are consistent with the proposition that growth of the mantles occurs in the galactic shock region predicted by the density-wave theory. Estimates of the total visual extinction toward the galactic center and the consequent estimates of the total amount of far infrared radiation are shown to depend critically on the grain model. Variations of the ratio of far ultraviolet to visual extinction are correlated with the conditions for growth of mantles on the bare small particles which are generally prevented from accreting mantles primarily because of their extreme temperature fluctuations produced by the ultraviolet photons in the radiation field.

We have investigated the gas-to-dust ratio in the Galaxy by comparing 21-cm Hicolumn densities with the color excesses of globular clusters. We find a constant gas-to-reddening ratio in interstellar clouds and the intercloud medium. This ratio is also independent of galactic latitude.

The absorption cross-sections of dust grains in the Lyman continuum region are calculated based on integrated spectra of OB star clusters and observed values of the IR excess radiation from H ii regions.