Context : Some well-studied Herbig Haro objects have associated with them one or more cold , dense , and quiescent clumps of gas .
We propose that such clumps near an HH object can be used as a general measure of clumpiness in the molecular cloud that contains that HH object .

Aims : Our aim is to make a survey of clumps around a sample of HH objects , and to use the results to make an estimate of the clumpiness in molecular clouds .

Methods : All known cold , dense , and quiescent clumps near HH objects are anomalously strong HCO ^ { + } emitters .
Our method is , therefore , to search for strong HCO ^ { + } emission as an indicator of a clump near to an HH object .
The searches were made using JCMT ( for Northern hemisphere objects ) and SEST ( for Southern hemisphere objects ) in the HCO ^ { + } ( 3–2 ) and also ( for SEST observations ) H ^ { 13 } CO ^ { + } ( 1–0 ) lines , with some additional searches for methanol and sulphur monoxide lines .
The sources selected were a sample of 22 HH objects in which no previous HCO ^ { + } emission had been detected .

Results : We find that half of the HH objects have clumps detected in the HCO ^ { + } ( 3–2 ) line and that all searches in H ^ { 13 } CO ^ { + } 1-0 lines show evidence of clumpiness .
All condensations have narrow linewidths and are evidently unaffected dynamically by the HH jet shock .

Conclusions : We conclude that the molecular clouds in which these HH objects are found must be highly heterogeneous on scales of less than 0.1Â pc .
An approximate calculation based on these results suggests that the area filling factor of clumps affected by HH objects is on the order of 10 % .
These clumps have gas number densities of \ga 3 \times 10 ^ { 4 } Â cm ^ { -2 } .