The knowledge of the the physical properties of the dust inside
cold condensations in a stage just before the collapse forming the
protostellar cores, is a key for the understanding of star formation process
and interstellar matter dynamics.  In particular, the opacity value is a
crucial parameter determined mainly by the dust grains absorptivity/emissivity
law.  The submillimeter balloon-borne experiment PRONAOS-SPM, has been
launched recently (september 22 - 23 1996).  It has discovered extended cold
condensations (T=12K, size from 0.1 to 1pc) in molecular cloud complexes such
as Orion A, Rho-Oph, M17, and a cold interstellar matter component inside
cirrus clouds.  This detection has been possible thanks to an unique
photometric sensitivity around a few MJy/sr/Hz^1/2.  Preliminary data
processing shows that the grain emissivity changes between these extended cold
condensations and the surrounding hotter regions.  We propose to perform two
kinds of observation toward the direction of the discovered new objects:  (1)
ISOPHOT mapping at 135 and 200 mu in order to increase the accuracy of the
dust temperature determination, (2) Full low resolution LWS spectral
observations in the direction of each typical extended cold condensation and
in the surrounding hotter region.  These spectra should be very efficient
tools to investigate the matter accretion around the grains, inside cold
condensation (shielded from UV radiation), and to compare the dust properties
between cold cores inside stellar forming regions and cold condensations in
diffuse medium.  In particular, detection of spectral features could be
crucial to:  i) study the grains mantle composition and growing processes, ii)
look for new carbon agregates such as those proposed by Marty et al.(1994).
Furthermore and independent of modelling forecasts, the accurate knowledge of
the emissivity/ absorptivity law (continuum and possible features) is a key
parameter for the understanding of the physical properties of the extended
cold condensations recently discovered with PRONAOS.